additions to our knowledge are so fundamental that we have become rapidly familiar with them, and easily forget how recently they have been added to our science. Many other illustrations of the newness of embryological knowledge might be given.
The methods used by von Baer and by all his successors in embryology down to 1860, or even later, were exceedingly simple. They worked almost entirely with fresh material, hand lenses, and sometimes with acetic acid to render the objects a little more transparent. The embryologists of that period were few in number, but they made many fundamental discoveries. I fancy that if the researchlings of our present luxuriously installed laboratories were put back into that time of lean resources, their publications would cease. As you know, the fundamental procedures in modern microscopical technique are the making of sections and the staining of them. The introduction of section cutting came about so gradually that its history seems to be lost to us. Many persons in the middle of the last century appear to have made sections by hand of various tissues. This was especially a practise among botanists. At first only fresh material was used, but it was learned that preserved material, especially that which had been properly hardened in alcohol, could be cut to greater advantage, and gradually the process of 'hardening' before cutting became more and more common. So long as the cutting was done only by hand with that favorite unsuitable instrument of old days, the razor, no very fine sections were possible, save occasionally by some person of exceptional dexterity. The first microtome, so far as known to me, was that devised by Professor His and employed by him, about 1866, for making serial sections of chicken embryos. Since then many inventors have contributed to the perfection of the instrument, and we now have the rather complex but very accurate and convenient automatic microtomes which are in such general use.
With the aid of microtomes, we can make perfect series of sections, and by mounting the entire series from a given object, it becomes possible to examine every part of it under the microscope. In the case of embryos serial sections are invaluable. We have been forming in my laboratory at the Harvard Medical School a collection of such series of sections of vertebrate embryos. The total number of series at the present writing is 1,106, of which forty-nine are from human embryos. The total number of sections is probably over 100,000. This collection has already served as the basis of forty-two embryological investigations and we trust that it will serve in the future for very many more. So far as I know the collection is unique in plan and extent. As soon as we are established in our superb new laboratory, into which we are about to move, we shall be glad to have
- Described in the Archiv für mikroskopische Anatomie, 1870.