GROWTH. 31j^ tion by saying that the indefinite stretching or flattening of a mass achieves the same thiug "as splitting it up into a great many sienilei- bars, and phicing these end to end, or, as splitting up the mass into a great man}' thin slices, and join- ing these edge to edge. In both cases a great gain of surface is accomplished. Now if a body branches rapidly, it docs the same thing as is done when it is split up into a great many slender bars; namely, it increases its surface in a ratio corrcs])onding exactly to the rate at which the splitting or branching takes place." This is what the vascular system has done. The branching of the blood-vessels, with their in- numerable ramifications, "is nothing more than a physiological response, developed in a geometri- cal ratio, to a correspondingly rapid increase in the volume of the organized matter to be metabolized." So with the nervous system, "the extreme attenuation of the terminal and central interrelative fibres given off by the ganglion cells of the nervous system means a gain of sur- face in proportion to mass which is nowhere else paralleled by any active constituent tissue of animal liodies." Tlie enormous development of the irritable surface is, he adds, correlated with the extreme irritability of the nervous system. This is exemplified by the tenacity of the nerve ter- minals of the auditory organs and the other oTgans of sense; while it is the most attenuated structures known to the histologist which are the most irritable, as cilia, flagella, etc. This subject is further illustrated by the stretching of plasma into threads, as the pseudopods of many protozoans, the branches and filaments or flattened leaf-like forms of gills, as well as the roots and branches of plants. Thus organisms higher than spherical protozoa have, after life began, proceeded, during their growth in volume, to develop surfaces according to this rule of geometrical progression. Growth as a Finction of Cells. As organ- isms are one or many-celled, and as the many- celled plants or animals arise from a single cell (the seed or egg), growth is primarily a function of cells. During growth the cells undergo rapid multiplication, which causes the body to increase in weight, this l)eing determined by the number of cells and their average weight. Increase in age affects the different cells iniequally, and hence some cells are dividing while others are not. In the mature body a minority of the cells are in process of division, while the majority are not. (Minot. ) Very few observations have been made as to measurement of the growth of animals. Semper measured that of the pond-snail; but those of B. A. Oould and of H. P. Bowditch on man are more extensive and reliable. In animals, according to ilinot, the rate of growth in weight for man, and for some few other animals, in- creases for about half the period of growth, then remains approximately constant for a short time, and finally slowly diminishes until it becomes zero. Factobs Concerned in Growth. These are chiefly external, such as abundance of food and other conditions of existence, as heat, light, etc. Hatschek assumes that in growth the simple molecule of living proteid continually attracts elements to itself from the food. Growth is more rapid in a well-fed animal or plant. Develop- ment of the higher organisms is usually intro- duced by a period of rapid cell-division with al- GROWTH. 7nost no growth. This is followed by a period of rapid imbibition of water during which the tis- sues are stretched, and growth occurs with great rapidity. Later still, the cell-walls thicken, in- tercellular substance is deposited, and the dry weight slowly increases. This is the special period of assimilation and secretion. In the de- velopment of the frog the jicrcentage of water rises to OG when half a month old. and tlicn falls slowly. At six weeks from fertilization the human embryo consists of 97.5 per cent, water, but at birth the proportion of water has fallen to 75 per cent., and in adult man it is GC per cent. The growth processes may be classified as 'transi- torj',' in which growth is followed by a rcturii to the former size, and 'permanent' or develop- mental, in which there is a persisting enlarge- ment which plays an important part in develop- ment. Transitory growth may be illustrated by the enlargement of those cells which cause the leaflets of the sensitive plant to turn; this en- largement is only temporary. Developmental growth is illustrated by the increase of the em- bryonic plant. Growth phenomena may also be classified as 'diffused' or 'localized.' DilTuscd growth affects the entire individual or many of its parts, whereas in localized growth the process is confined to a limited region. In early develop- ment the wdiole animal swells by diffused growth ; later, the separate organs enlarge as a result of localized growth. Although cell-divisi(m usually accompanies growth, it is in no way the cause of growth. The curve of growth is obtained by join- ing with a line the tops of ordinates erected from a horizontal base-line at equal time intervals, the height of the ordinates being proportional to the volume of the organism. A study of these growth-curves shows that absolute increments of size are at first small, rapidly increase to a maximum, and then decline to zero. This decline to zero is not a necessary property of protoplasm, but it is an adaptation to the limited size which is advantageous to most animals. Various external agents influence the rate of growth as follows: (1) Chemical Agents. — The assimilative and secretory are chemical processes. They require certain materials to work upon.' These materials are the food of organisms. The principal elements found in the body are car- bon, oxygen, nitrogen, calcium, phosphorus, potas- sium, sodium, chlorine, magnesium, sulphur, silicon, and iron, which enter the body in various combinations, while each plays a definite role. Since phosphorus is especially abundant in embryonic tissue, its peculiar importance for. growth is indicated. Potassium is probably of great importance in imbibition. Iron is essen- tial in the early processes of cell-division. Be- sides the inorganic elements, organic food is essential to the growth of animals; various chemical agents that are not built up into the organic body may accelerate growth by acting as a stinuilus. Certain poisons do this. (2) Wafer. — The imbibition of water is an important growth, process. Developing embryos are usually found in moist situations. (.■?) Density of the Medium. — The rapidity with which water is imbibed depends largely upon the osmotic pressure of the medium, and this is determined by its density. Regeneration of marine animals is hastened in dilute sea-water, retarded in a concentrated solution. Pond-or-
