8 INTR OD UCTION 



the body of which it forms a part, since the cellular structure of living 

 things was not comprehended until nearly two centuries later, the 

 spermatozoon was still undiscovered, and the nature of fertilization 

 was a subject of fantastic and baseless speculation. For a hundred 

 years after Harvey's time embryologists sought in vain to penetrate 

 the mysteries enveloping the beginning of the individual life, and 

 despite their failure the controversial writings of this period form one 

 of the most interesting chapters in the history of biology. By the 

 extreme " evolutionists " or " praeformationists " the Q.gg was believed 

 to contain an embryo fully formed in miniature, as the bud contains 

 the flower or the chrysalis the butterfly. Development was to them 

 merely the unfolding of that which already existed ; inheritance, the 

 handing down from parent to child of an infinitesimal reproduction 

 of its own body. It was the service of Bonnet to push this concep- 

 tion to its logical consequence, the theory oi'ejnboitemejtt or encase- 

 ment, and thus to demonstrate the absurdity of its grosser forms, 

 pointing out that if the egg contains a complete embryo, this must 

 itself contain eggs for the next generation, these other eggs in their 

 turn, and so ad infinitinn, like an infinite series of boxes, one within 

 another — hence the term eruboitement. Bonnet himself renounced 

 this doctrine in his later writings, and Casj^ar Friedrich Wolff (1759) 

 led the way in a return to the teachings of Harvey, showing by pre- 

 cise actual observation that the Q,gg does not at first contain any 

 formed embryo whatever ; that its structure is wholly different 

 from that of the adult ; that development is not a mere process 

 of unfolding, but involves the continual formation, one after an- 

 other, of new parts, previously non-existent as such. This is some- 

 what as Harvey, himself following Aristotle, had conceived it — 

 a process of epigenesis as opposed to evolution. Later researches 

 established this conclusion as the very foundation of embryological 

 science. 



But although the external nature of development was thus deter- 

 mined, the actual structure of the Q.%g and the mechanism of inheri- 

 tance remained for nearly a century in the dark. It was reserved 

 for Schwann (1839) "^^^ his immediate followers to recognize the 

 fact, conclusively demonstrated by all later researches, that tJie egg 

 is a cell having the same essential structure as other cells of the 

 body. And thus the wonderful truth became manifest that a single 

 cell may contain within its microscopic compass the sum-total of 

 the heritage of the species. This conclusion first reached in the 

 case of the female sex was soon afterward extended to the male 

 as well. Since the time of Leeuwenhoek (1677) it had been known 

 that the sperm or fertilizing fluid contained innumerable minute 

 bodies endowed in nearly all cases with the power of active move- 



