EMBRYOLOGY AND MEDICAL PROGRESS 7 



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 em- 

 bryology down to 1860, or even later, were exceedingly simple. They 

 worked almost entirely with fresh material, hand lenses, and some- 

 times 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, 2 

 for making serial sections of chicken embryos. Since then many in- 

 ventors have contributed to the perfection of the instrument, and we 

 now have the rather complex but very accurate and convenient auto- 

 matic 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 form- 

 ing in my laboratory at the Harvard Medical School a collection of 

 sl^ch 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 



2 Described in the Archiv fiir mikroskopische Anatomie, 1870. 



