Popular Science Monthly/Volume 71/September 1907/The Problem of Age, Growth and Death III
|THE PROBLEM OF AGE, GROWTH AND DEATH|
JAMES STILLMAN PROFESSOR OF COMPARATIVE ANATOMY IN THE HARVARD MEDICAL SCHOOL
III. The Rate of Growth
Ladies and Gentlemen: In the first of the lectures, I described those grosser characteristics of old age, which we ourselves can readily distinguish, or which an anatomical study of the body reveals to us. In the second lecture I spoke of the microscopic alterations which occur in the body as it changes from youth to old age. But besides the changes, which we have already reviewed, there are those others, very conspicuous and somewhat known to us all, which we gather together under the comprehensive term of growth. It is growth which I shall ask you to study with me this evening, and I shall hope, by the aid of our study, to reinforce in your minds the conclusion which I have already indicated, that the early period of life is a period of rapid decline, and that the late period of life is one of slow decline.
In order to study growth accurately, it is desirable, of course, to measure it, but since we are concerned with the general problem of growth, we wish no partial measure, such as that of the height alone would be. And indeed, if we take any such partial measure, how could we compare different forms with one another? The height of a horse is not comparable to that of a man; the height of a caterpillar is not comparable to that of any vertebrate. Naturally, therefore, we take to measuring the weight, which represents the total mass of the living body, and enables us at least with some degree of accuracy to compare animals of different sorts with one another. Now in studying this question of the increase of weight in animals, as their age increases, it is obviously desirable to eliminate from our experiments all disturbing factors which might affect the rate of growth or cause it to assume irregularities which are not inherent either in the organization of the animal or in the changes age produces. The animals which belong to the vertebrate sub-kingdom, of which we ourselves are members, can be grouped in two large divisions according to the natural temperature of their bodies. The lower vertebrates, the fishes, frogs and their kin, are animals' which depend for their body temperature more or less on the medium in which they live. The other division of vertebrate animals, which includes all the higher forms, are so organized that they have within certain limits the power of regulating their own body temperature. Now it is easily to be observed—and any one who has made observations upon the growth of animals can confirm this—that animals otherwise alike will grow at different speeds at different temperatures. There are animals, like the frogs and salamanders, which will live at a Fig. 19. Four Tadpoles of the European Frog Rana fusca. After Oskar Hertwig. The four animals are all of the same age (three days) and raised from the same batch of eggs, but have been kept at different temperatures.
|A at 11.5° Centigrade||B at 15.0° Centigrade|
|C " 20.0°"||D " 24.0°"|
all seasons of the year, in great abundance, and at small expense. The animals themselves being of moderate size do not, of course, require such extraordinary amounts of food as the large animals, should we experiment with them. Accordingly with guinea-pigs I began making, years ago, a long series of records, taking from day to day, later from week to week, and then, as the animals grew older, month by month, the weight of recorded individuals. There was thus obtained a body of statistics which rendered it possible to form some idea of the rapidity of growth of this species of mammal.
Now in regard to the rapidity of growth, it is necessary that we form clearer notions than perhaps you started out with when you came into the hall this evening. I will ask for the next of our pictures on the screen, where we shall see illustrated to us older methods of recording the progressive growth of animals. This is a chart taken from the records of my friend, Dr. Henry P. Bowditch, showing the growth of school children in Boston. Here we have, in the lower part of the figure, the two curves of growth in weight. The upper curve if; the weight of boys. We can follow it back through the succession of years down to the age of five and one half years, when the records begin. The child weighs, as you see, a little over forty pounds at that time. When the boy reaches the age of eighteen and one half years, he approaches the adult size, and weighs well over 130 pounds. Here then we see growth represented to us in the old way, the progressive increase of the animal as it goes along through the succession of years. Now this is a way which records the actual facts satisfactorily. It shows the progressive changes of weight as they really occur; but it does not give us a correct impression of the rate of growth. Concerning the rate of growth, some more definite notion must be established in our minds before we can be said to have an adequate conception of the meaning of that term. It is from the study of the statistics of the guinea-pigs, and of other animals, which I have since had an opportunity of experimenting with, that we get indeed a clearer insight as to what the rate of growth really is and really means.
I should like to pause a moment to say that when I first published a paper upon the subject of growth, it, fortunately for me, interested the late Dr. Benjamin Gould. The experiments which I had made and recorded in that first publication came to a sudden end, owing to a disaster for which I myself was personally not responsible, by which practically my entire stock of animals was suddenly destroyed. Dr. Gould, after consulting with me, proposed that I should have further aid from the National Academy of Sciences, and through his intervention I obtained a grant from the Bache fund of the academy. That liberal grant enabled me to continue these researches, and this is the first comprehensive presentation of my results which I have attempted. In this and the subsequent lectures, I hope that enough of what is new in scientific conclusions may appear to make those to whose generosity I am indebted feel that it has been worthily applied. I can not let such an occasion as this pass by without expressing publicly my gratitude to Dr. Gould for his encouragement and support at a time when I most keenly appreciated it.
If animals grow, that which grows is of course the actual substance of the animal. Now we might say that given so much substance there should be equal speed of growth, and we should expect, possibly, to find that the speed would be more or less constant. I can perhaps illustrate my meaning more clearly, and briefly render it distinct in your minds, by saying that if the rate of growth, as I conceive it, should remain constant, it would take an animal at every age just the same, length of time to add ten per cent, to its weight; it would not be a question whether a baby grew an ounce in a certain length of time, and a boy a pound in the same time, for the pound might not be the same percentage of advance to the boy that the ounce would be to the baby. In reality with an advance of an ounce the baby might be growing faster than the older boy with the addition of the pound.
In the next slide which we are to have thrown upon the screen we have my method of measuring rate of growth illustrated graphically. There is here a curve which represents the rate of growth of male guinea-pigs. The figures at the bottom indicate the age of the animals in days. When guinea-pigs are born, they are very far advanced in development, and the act of birth seems to be a physiological
shock from which the organism suffers, and there is a lessening of the power of growth immediately after birth. But in two or three days the young are fully recovered, and after that restoration they can add over five per cent, to their weight in a single day. But by the time they are 17 days old, as represented by this line, they can add only four per cent., and by the time they are 24 days old, less than two per cent.; at 45 barely over one per cent.; at 70 still over one per cent.; at 90 less; at 160 less; and towards the end the curve continues dropping off, coming gradually nearer and nearer to zero, to which it closely approximates at the age of 240 days. In about a year, the guinea-pig attains nearly its full size. You notice that this curve is somewhat irregular. Such is very apt to be the result from statistics when the number of observations is not very large. It means simply that there was not a sufficiently large number of animals measured to give an absolutely even and regular set of averages. But the general course of the curve is very instructive. In the earlier condition of the young guinea-pig there is a rapid decline; in the later, a slow decline. The change from rapid to slow decline is not sudden, but gradual, as you see by the general character of this curve.
In the next slide we can see immediately that what I have asserted as true of the male is equally true of the female, although the values
which we have differ slightly in the two sexes, and there are accidental but not significant variations in this curve as in the first. Here also we observe at once an early period of rapid decline in which the rate of growth is going down and down—a period of slight decline in which, to be sure, it is going down still, but with diminished rapidity.
There is another method by which we can represent this change
in the rate of growth which will perhaps help to illustrate it; and in the next of our pictures we see this other form of representation before us. This vertical line represents the length of time which it takes a young male guinea-pig to add ten per cent, to its weight the first time. Here the third time—the fourth—the fifth—and you see as it is growing older and older it takes the animal longer and longer to add ten per cent, to its weight. Finally we get to the nineteenth addition, and we see that the period is very long indeed. How long that period is we can judge by the figures here upon the left, which represent the length of the days. From the base line to this one marked "ten" is a period of ten days, and you see from the time the guinea-pig has added to its weight ten per cent, for the nineteenth time it does it so slowly that it requires ten days and more; for the twenty-first time, nearly twenty; for the twenty-second time, nearly forty. Here where the number of observations becomes small, the curve grows very irregular. Thus we demonstrate that as the animal grows older it takes longer and longer to add ten per cent, to its weight. In the other sex, as the next slide shows, the same phenomena can be clearly demonstrated; here are the periods as before, lengthening out, as you see, at first; then becoming very long indeed. In the following slide I have another
form of representation of this same phenomenon as it occurs in the human subject. Here is a diagram of growth, which represents, as accurately as I could determine it, the curve complete for man from the date of birth up to the age of forty years. It has been calculated by a simple mathematical process where these ten-per-cent. increments fall, and from each point in this curve where there has been such an increment, a vertical line has been drawn, as you see here. These lines are very close together at the start. One ten per cent, after another follows in a short interval of time, but gradually the time, as indicated by the space between two of these vertical lines, increases, and when the individual is three years old, you can see there has been a very great
lengthening out of the period which is necessary for it to add ten per cent, to its weight. Then it comes at the age of twelve to a period of slightly more rapid growth, a fluctuation which is characteristic of man, but does not appear in the majority of animals. After that comes very rapidly the enormous lengthening of the period; and I have not added the last ten per cent, because the curve here at the top, you see, is not very regular, and it could not be calculated with certainty. Our diagram is merely another form of graphic representation of the fact that the older we are the longer it takes us to grow a definite proportional amount.
The next slide carries us into another part of our study, away from the mammals which we have thus far considered, into the class of birds. The growth of chickens is represented here. Now a chicken is born in a less matured state than a guinea-pig, and has a good deal
higher efficiency of growth at first. In a chicken, as in a guinea-pig, birth is a disturbing factor, and growth immediately after the hatching of the chicken is a little impeded, but the chick quickly recovers and, as we see, the first time when the rate can be distinctly measured we get a nine-per-cent. addition to the weight in a single day. In a chicken as in the guinea-pig, the rate gradually diminishes. The change from the rapid decline at first to the later slower decline is more gradual; the curve is more distinctly marked in the chicken as a round curve. There is not in the bird so marked a separation of the preliminary rapid decline and the later slower decline as we find in the guinea-pig. The curve again is very irregular because I had only a very limited number of observations upon the weight of chicks. The other sex, as the next slide will show, presents similar phenomena, though the female chickens do not grow quite as fast as their brothers. Here we notice an increaseof almost, but not quite nine per cent., rapidly falling down so that after the chick is two months old it never adds as much as three per cent, to its weight. It loses in the first two months from a capacity to add nine per cent., down to a capacity of adding less than three. It loses in two months two thirds of its total power of growth, for from nine to zero is divisible into two parts, of which the first, from nine down to three, would be two thirds, and the second, from three to zero, would be one third. Here then we learn that two thirds of the decline which occurs in the life of a chick takes place in two months, and for the rest of the life of the bird there is a decline of one third. That, you must acknowledge, is an extraordinary and most impressive difference. If it be true that the more rapid growth depends upon the youth of the individual,—its small distance in time from its procreation, then we may perhaps, by turning to other animals which are born in a more immature state, get some further insight into these changes; and that I have attempted to do by my observations upon the development of rabbits. Rabbits, as you know, are born in an exceedingly immature slate. They are blind, they are naked, they are almost incapable of definite movements, quite incapable of locomotion, and are hardly more than little imperfect creatures lying in the nest and dependent utterly upon the care of the mother, quite unable to do anything for themselves except take the milk which is their nourishment. They are indeed animals born in a much less advanced stage than are the guinea pigs. Upon the screen we see this interesting result demonstrated to us, that a male rabbit, the fourth day after its birth, is able to add over seventeen per cent, to its weight in one day. From that the curve drops down, as you see, with amazing rapidity, so that here at an age of twenty-three days the rabbit is no longer able to add nearly eighteen per cent, daily, but only a little over six. At the end of two months from its birth, the growth power of the rabbit has dropped to less than two per cent., and at two months and a half it has dropped to one. The drop in two and a half months has been from nearly eighteen per cent, down to one per cent., and the rest of the loss of one per cent, is extended over the remaining growing period of the rabbit. Could we have a more definite and certain demonstration of the fact that the decline is most rapid in the young, most slow in the old? It is not in this case any more than in the others the one sex that demonstrates this fact, for in the female we find exactly the same phenomena, as the next slide will show. The irregularities are not significant. The strange dip at thirty-eight days, for instance, corresponds to an illness of some of the rabbits which were measured, but they rapidly recovered from it and grew up to be fine, nice rabbits. If instead of measuring half a dozen rabbits, we had measured two hundred or five hundred, these irregularities would certainly have disappeared. The females in the case of the rabbits, as in the case of the guinea-pigs, are not able
to grow quite so fast at first. We see here sixteen instead of over seventeen per cent, as the initial value, but the general character of the drop is the same, enormously rapid at first and very slow afterwards. All of our cases, then, show the same fundamental phenomena appearing with different values.Now in regard to man, we do not possess any such adequate series of statistics of growth as is desirable. We have many records of the weight of babies, by which I mean children from the date of birth up to' one year of age. We have also very numerous records of school children, which will extend perhaps from five and one half up to say seventeen, eighteen or even nineteen years. There are records of boys
at universities, and a still more limited number of weighings of girls at colleges. But all these statistics piled together do not give us one comprehensive set of data including all ages. This is very much to be regretted, and it would be an important addition to our scientific knowledge could statistics of the growth of man be gathered with due precautions. It would fill one of the gaps in our knowledge which is lamentable. We have, however, some rough, imperfect data which for our present purposes it seems to me are adequate, and the results of the study of these will be shown by the next series of pictures.
But let us pause for a moment to consider this singular table. It shows in this column the number of days which it takes for each species
|Species||Days needed to
|100 Parts Mother's Milk Contain|
of animal indicated at the left to double its weight after birth. A man requires 180 days to double his weight; a horse, 60; a cow, 47; a goat, 19; a pig, 18; a sheep, 10; a cat, 91⁄2; a dog, 8; a rabbit, 6 (or possibly 7 days). Now here are analyses of the milk. The main point of interest is to be found in the figures in this column, which represent the amount of albuminoid, or proteid material contained in the milk. You will observe that for man the proportion is lowest, 1.6 per hundred parts; the horse has a little more—2; cattle—3.5; and so the values run. In other words, it is obvious that the less the proteid in the milk, the longer does the species require to double its weight. This looks at first sight as if there were a relation between the composition of the milk and the period of growth of the animal; but you know very well that if you take the milk of a cow, which is very much richer in proteid material, and feed it to a baby, a human baby, that baby does not grow at the same rate as the young cow, but grows at the human rate. It is obvious, therefore, that it is somewhat more complicated than a mere question of food supply. We have in fact one of the beautiful illustrations of the teleological mechanism of the body. These various species have their characteristic rates of growth, and by an exquisite adaptation, the composition of the mother's milk has become such that it supplies the young of the species each with the proper quantum of proteid material which is needed for the rate of growth that the young offspring is capable of. It is a beautiful adjustment, but there is not a causal relation between proteid matter and this rate of growth. It is an example of correlation, not of causation.
We pass now to the next of our slides, which carries us over into the study of our own species. It is not possible at the present time to represent in any form of curve which I have seen the daily percentages of increment for man covering the whole period of growth. In order to get the results together, I have confined myself here to the representation of the yearly percentages. Now from the age of zero to the age of one year, you see according to this table a child is able to increase its weight 200 per cent. But from the beginning of the first to
the end of the second year, only 20 per cent., and thereafter it fluctuates in the neighborhood of 10 per cent, a year until the age of 13. At 14 or 15 there is a fluctuation, an increase, and then the decline goes on again and slowly we see the growth power fading out. Authors are not agreed as to the exact statistical value, and so I will ask to have thrown upon the screen another curve, also representing the percentage increase of boys, and based chiefly upon English tables. For these data I am indebted to my friend Professor Donaldson, of the Wistar Institute in Philadelphia. He finds in these records an increment of a little more than 200 in the first year, but the drop comes during the second year and is startling in its enormous extent and is contrasted with the later less decline. The phenomena may well arouse our attention and convince us that we are approaching a most important scientific question, the question of why the drop comes in this way. In the case of girls, as the next of our slides will show, we can prove the same phenomena with slightly different values. Girls, like the females of other species, grow a little less forcibly, so to speak, than boys. They do not quite
attain a 200 per cent, value for the first year, but they too drop in a similar manner to the boys to about 30 per cent., and away down towards 10 per cent, in the third year. Then comes the long slow gradual decline up to the period of twenty-three. Professor Donaldson, as our next slide will demonstrate to us, has prepared curves from the English figures for girls also. They come up nearer to the 200 per cent, than in Mühlmann's table, but drop well below 30 per cent, in the second year, and down to 20 per cent, in the fourth. Then occurs the slight increase of growth in the period of twelve, thirteen, fourteen years, and next the final stage of decline. In the four cases the human rate curve is similar. The great fall takes place at the beginning, the slow fall towards the end. Professor Thoma has thought he could get somewhat more accurate results by putting boys
This completes the series of curves which I had prepared to present to you to show the rate of growth in animals from their birth only, but of course there has been also a growth of the animals which preceded their birth, and that now must briefly be considered.
The mere inspection of developing embryos of known ages gives us some idea of the rate of growth. With the aid of the lantern I will ask you to look with me at some pictures of the developing chick and developing rabbit. Let us begin with the chick.
We have first an embryo of twenty hours of incubation; following it one of one day. You can observe just a little line of structure indicated and showing where the longitudinal axis is to be situated. By the second day the chick has distinctly a head and a little heart, and those who are expert can differentiate with a microscope the axis of the body, the beginning of the formation of the intestine and of the muscles. At the end of the first day there was little more than a mere gathering of cells, but during the twenty-four hours of the second day the gathering has changed from a mere streak upon the surface of the yolk to a well-formed individual, with recognizable parts and several times the volume it had when one day old. The next figure illustrates the alteration which occurs during, approximately, the third day. It is obvious that the embryo has again made an enormous increase in volume. The eye has developed, the heart has become large, the tail is projecting, the dorsal curve of the future neck is distinguishable. We pass next to the fourth day. Is it not a strange looking beast, with its wing here and leg there, a little tail at this point; an enormous eye, almost monstrous in proportion; and, finally, here a bit of the brain. After five days we have a chick the brain of which is swelling, causing the head to be of so queer a shape that, with the eye, which seems out of all proportion to the rest of the body, it imparts an uncanny look to the embryo. The wing is shaping itself somewhat, and the ends of the leg, we can see, will, by expansion, form a foot. Finally, the chick after seven and after eight days is figured. In the short interval of only six days the chick grows from the size represented by Fig. 2 to that shown in the last figure upon the plate. It is an enormous increase. Suppose a chick after it was born were to grow at such a rate as that! The eight-day embryo is thirty or forty times as big as it was eight days before. It would seem marvelous to us if a chick after it was hatched should become in eight days thirty times as large and heavy as when it first came out from the egg. It is perhaps advisable to let you follow the growth of the chick a little farther, and accordingly I present another picture which shows an embryo of about ten days. The little marks upon the surface of these embryos indicate the commencing formation of the feathers. A comparison of the series of figures proves that the development is taking place with marvelous speed. We need only to look at these stages, comparing them with one another, to realize that the progress of the embryo in size and development occurs with a rapidity which is never to be found in later stages.
The history of embryonic rabbits declares with equal emphasis that the earliest development is extremely rapid. I wish now to show you
a series of pictures to illustrate in the same manner the progressive development of the rabbit. Numbers one to five of the figures upon the screen represent what is known as the germinal area, in the center of which the actual embryo is gradually formed. In No. 1 merely the axis is indicated, in front of and alongside of which the parts of the embryo are to arise, as is suggested by Nos. 2, 3, 4, 5, These stages cover the seventh and eighth days. Nos. 6 to 14 figure actual embryos, No. 6 of nine and a half, No. 14 of fifteen days. No. 6 is singularly twisted into a spiral form, the reason for which is still undiscovered. No. 9 shows the condition at eleven days—notice the limbs, a leg in front and a leg behind, each only a small mound as yet upon the surface
of the body; the distinct eye, the protuberance caused by the heart. Nos. 11 and 12 show the embryonic shape at twelve and a half and at thirteen days—there has been a great increase of size with accompanying modifications of form. The next pair, Nos. 13 and 14, present us embryos of fourteen and fifteen days, respectively, and you see that the growth is very marked indeed, and the change of form obvious; the creature is now changing from the embryonic type into something resembling a rabbit. Other pictures could readily be added, but, though two weeks must still elapse before the animal will be ready to enter the world, it is not necessary for my present purpose to include this period in our survey. We need only contemplate, it seems to me, the series of drawings in Fig. 37 to realize that the early embryonic growth of the rabbit, like the embryonic growth of the chick, proceeds with a speed which is never paralleled by the growth during later stages.
Fig. 37. Fourteen Stages of the Developing Rabbit, after Minot's and Taylor's "Normal Plates." All the figures are magnified four diameters. Nos. 2 to 5 are irregular as to age, but show successive stages of development. The early development is extremely variable and the observations do not yet suffice to determine the average typical condition for each day under nine.
|No.||1.||Embryo of||7||1⁄2||days.||No.||8.||Embryo of||10||1⁄2||days.|
Now I had a considerable number of rabbit embryos preserved in alcohol, and though it was not very accurate to weigh them as alcoholic specimens, in order to determine their true weight, yet I resolved to do so as it was the best means at my disposal at the time. The result of that weighing was very interesting to me, because it showed that in the period of nine to fifteen days the rabbit had, on an average, added 704 per cent, to their weight daily; but in the period of from fifteen to twenty days, the addition is very much less than this, only 212 per cent. But these rabbits at ten days have already had a considerable period of development behind them, and as we have discovered that the younger the annual the more rapid its growth, we are safe, it seems to me—since we have learned that from the tenth to the fifteenth day there is a daily increase of over 700 per cent.—in assuming that in yet younger rabbits an increase of a thousand per cent, per day actually occurs. That is not so extraordinary an assumption, for bacteria are known to divide every half hour, and if the little bacterium divides and grows up to full size in half an hour, and then divides again, it means that within a half hour one bacterium has become two, and has increased, obviously, 100 per cent.; and if those two again divide as before, we should have four bacteria at the end of an hour—an increase of 400 per cent., and at the end of another half hour, of 800 per cent., and so on ever in geometrical progression. We learn, then, that bacteria may in a few hours add 1,000 per cent, to their original weight, and it is not by any means an exorbitant demand upon our credulity to accept the conclusion that in their early stages, rabbits and other mammals and birds are capable of growing at least 1,000 per cent, a day. If this be true, and it doubtless is true, we can adopt it as a convenient basis for comparison. As we learned from the rate curves, which were projected upon the screen earlier during the hour, the male rabbit gains in one day immediately after birth nearly eighteen per cent.—seventeen and four tenths per cent.—and the female rabbit gains nearly seventeen per cent. Now we can estimate the loss very simply by deducting this rate, which is the capacity of the animal to grow persisting at birth, from its original capacity, which we assume to have been 1,000 per cent, per day. And if we do that the result is obvious. Over 98 per cent, of the original growth power of the rabbit or of the chick has been lost at the time of birth or hatching, respectively, and the same thing is equally true of man. We start out at birth certainly with less than two per cent, of the original growth power with which we were endowed. Over 98 per cent, of the loss is accomplished before birth—less than two per cent, after birth. That, I think is a rather unexpected conclusion, certainly not one which, until I began to study the subject more carefully, I in the least expected; and even now when I have become more familiar with it, it still fills me with astonishment, it is so different from the conception of the process of development as we commonly hold it, from our conclusions based on our acquaintance with the growth and progress of the individuals about us. We overlook the fact that the progress which each individual makes is the result of accumulation. It is as if money was put into the savings-bank; it grows and becomes larger, but the rate of interest does not alter. So too with us; we see there is an accumulation of this wealth of organization which gives us our mature power. But as that accumulation goes on, our body seems to become, as it were, tired. We may Fig. 38. compare it to a man building a wall. He begins at first with great energy, full of vigor; the wall goes up rapidly; and as the labor continues fatigue comes into play. Moreover, the wall grows higher, and it takes more effort and time to carry the material up to the top of the wall, and to continue to raise its height, and so, as the wall 400% grows higher and higher, it grows more slowly and ever more slowly, because the obstacles to be overcome have increased with the very height of the wall itself. So it seems with the increase of the organism; with the increase of our development, the obstacles to our growth increase. How that is I shall hope to explain to you a little more clearly in the next lecture.
We have one more slide, which I would like to show you. It indicates the rate of growth in man before birth as far as it can be indicated without better knowledge. The time intervals in the diagram correspond to the so-called lunar months—the ten lunar months of prenatal life. Of our early development we know very little so far as statistics are concerned, but from the third month onward we have some records. It is found that from the third to the fourth month the increase is 600 per cent. Just contrast that with 200 per cent, added in one year after birth; 600 per cent, in one month against 200 per cent, in one year. From the fourth to the fifth month it is scarcely over 200 per cent. It then becomes only a little more than 100. In the' seventh month, less than 100; and finally in the ninth and tenth months, it becomes very small indeed, less than 20, so that during the prenatal life of man, as we have seen in the prenatal life of the rabbit and of the chick, the decline in the power of growth is going on steadily all the time.
I shall use the few remaining moments to report to you yet another bit of evidence of the originally enormous power of growth. It has been estimated that the germ of the mammal, with which the development commences, has a weight of 0.6 milligram; another estimate which I have found is of 0.3 milligram. Perhaps I can give you some idea of what this value means by telling you that if the weight of the original germ of a mammal is assumed to be 0.6 milligram, we could, according to the laws of the United States, send 50,000 such germs by letter postage for two cents. It would take 50,000 germs to make the weight of one letter. That perhaps will give you some impression of the extreme minuteness of the primitive germ. In the human species at the end of even a single month it is no longer merely a germ, but a young human being, very immature, of course, in its development, but already very much larger. I doubt—even after all that I have said this evening about the startling figures of growth for the earlier stages,—I doubt if you are prepared for the fact that the growth of the germ up to the end of the first month represents an increase of over a million per cent. How much over a million per cent, we can not calculate accurately, because we do not know accurately the weight of the original germ, but an increase of a million per cent, is not above the true value. Contrast that with anything which occurs in the later periods. What a vast change has happened! What an immense loss has taken place! The rate of this loss is evidently diminishing. The less occurs with great rapidity in the young—less rapidity the older we become. I attempted to convince you in the first and second lectures that that which we called the condition of old age, is merely the culmination of changes which have been going on from the first stage of the germ up to the adult, the old man or woman. All through the life these changes continue. The result is senility. But if, as the phenomena of growth indicate to us so clearly, it be true that the decline is most rapid at first, then we must expect from the study of the very young stages to find a more favorable occasion for analysis of the factors which bring about the loss in the power of growth and change as the final result of which we encounter the senile organism. Not from the study of the old, therefore, but from the study of the very young, of the young embryo, and of the germ, are we to expect insight into the complicated questions which we have begun to consider together. I shall hope in the next lecture to prove to you that the supposition which has guided my own observations is correct, and to be able to show you that we do actually, from the study of the developing embryo, glean some revelations of the cause of old age.
- After Abderhalden, Zeitschrift für Physiologische Chemie, Band XXVI., p. 497.
- During the lectures a series of lantern slides-were projected upon the screen, made from photographs of mounted specimens of chicken embryos, which showed very clearly the progress of development in the chick during the very early stages. The first figure illustrated a chick of 18 hours' incubation. The embryo had been skimmed off from the surface of the egg, hardened, colored artificially and mounted in the manner of the ordinary microscopical preparation in Canada balsam. At this age the naked eye can just distinguish a line, which indicates the position of the axis of the embryo. The unaided eye can recognize nothing more. In the second picture the head and neck of the embryo were easily distinguishable, and a few of the earliest primitive segments. The third slide showed a stage of a day and a half. The spinal cord and brain were distinctly differentiated, and numerous so-called "blood islands" scattered about. The final slide of the series showed a chick of three and one half days. It has not seemed necessary to reproduce these figures with the present text, as they merely duplicate, on a larger scale and with more detail, the pictures which have been included.
- These estimates refer to the placental mammals only.