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SCIENCE. 



[N. S. Vol. XXI. No. 530. 



cussion of some latter day problems of em- 

 bryology and cytology I shall endeavor not 

 to violate the spirit of this tradition. The 

 task is not an easy one, owing to the com- 

 plexity of the data and their strangeness 

 to those who have not closely followed the 

 details of modern biological work; yet I 

 am encouraged to make the attempt by the 

 belief- that the problem of development be- 

 longs to those larger scientific questions 

 that are of enduring interest to all students 

 of nature. It is only fair to point out, 

 however, that a consideration of recent ad- 

 vances in this subject necessarily and 

 speedily leads us into a region that lies 

 remote from everyday experience, sur- 

 rounded by arid wastes of technical de- 

 tail, and inhabited by folk who speak an 

 uncouth foreign tongue. With the best 

 of intentions, therefore, the native guide 

 and interpreter has need of some forbear- 

 ance on the part both of his countrymen 

 and of the outlanders whom he attempts 

 to lead. 



I need not dwell on the absorbing, almost 

 tantalizing, interest with which the prob- 

 lem of development has held the attention 

 of naturalists from the earliest times. 

 Twenty centuries and more have passed 

 since Aristotle first endeavored to trace 

 something like a rough outline of its solu- 

 tion. The enormous advances of our 

 knowledge during this long period have 

 taken away nothing of the interest or fresh- 

 ness of the problem ; they have left it, in- 

 deed, hardly less mysterious than when the 

 father of science wrote the first treatise on 

 generation. I will not dwell on the epoch- 

 making work of Harvey, Wolf¥ and von 

 Baer, or the curious, almost grotesque con- 

 troversies of the eighteenth century, when 

 embryology invaded the field of philosophy 

 and even of theology. I will only point out 

 that even at that time, when embryology 

 was almost wholly limited to the study of 

 the hen's egg, embiyologists were already 



occupied with two fundamental questions, 

 which still remain in their essence with- 

 out adequate answer, and though metamor- 

 phosed by the refinements of more modern 

 observation and experiment still stand in 

 the foreground of scientific discussion. 

 The first of these is the question of pre- 

 formation versus epigenesis — whether the 

 embryo exists preformed or predelineated 

 in the egg from the beginning or whether 

 it is formed anew, step by step, in each 

 generation. The second question is that of 

 mechanism versus vitalism — whether de- 

 velopment is capable of a mechanical or 

 physico-chemical explanation, or whether 

 it involves specific vital factors that are 

 without analogy in the non-living world. 

 It is especially to some modern aspects of 

 these two questions that I invite your at- 

 tention ; and I shall also consider briefly 

 their relation to recent conclusions affect- 

 ing our theories of heredity and evolution. 



Let us first seek to define more clearly the 

 meaning of our terms. The embryologists 

 of the pre-Darwinian period, unhampered 

 by historical conundrums, fixed their atten- 

 tion on the single objective problem of the 

 nature of the germ and its mode of develop- 

 ment. The hen's egg contains something 

 which, though not visibly a bird or even 

 an embryo, will when maintained at a tem- 

 perature of about 37° C. for 21 days cause 

 a living chick to step forth from the shell. 

 What is that something and what manner 

 of machinery (if machinery it be) is set 

 in motion to work such a marvel ? The 

 early embryologists found no real answer 

 to this question. They determined the fact 

 that at the beginning the egg contains 

 nothing even remotely resembling a bird ; 

 that as early as the second day a rudely 

 fashioned embryo is visible in the egg; and 

 that day by day, as the incubation proceeds, 

 this embryo becomes more complex. The 

 bird appears to be progressively created out 

 of something that is without form and void 



