6 INTRODUCTION 



the influence of a mysterious " calidniii iuuatiiiu,'' out of not-living 

 matter. Whitman, too, in a recent brilHant essay,i has shown how- 

 far Harvey was from any real grasp of the law of genetic continuity, 

 which is well characterized as the central fact of modern biology. 

 Neither could the great physiologist of the seventeenth century have 

 had the remotest conception of the actual structure of the egg. 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 specu- 

 lation. 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 writ- 

 ings of this period form one of the most interesting chapters in the 

 history of biology. By the extreme "evolutionists" or " praeforma- 

 tionists" the &gg was believed to contain an embryo fully formed in 

 miniature, as the bud contains the flower or the chrysalis the butter- 

 fly. 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 conception to its logical consequence, the 

 theory of cviboitcmcnt or encasement, and thus to demonstrate the 

 absurdity of its grosser forms ; for 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 infinitiiui, like an infinite series 

 of boxes, one within another — hence the term " emboitement." 

 Bonnet himself renounced this doctrine in his later writings, and 

 Caspar Frederich Wolff (1759) led the way in a return to the teach- 

 ings of Harvey, showing by precise actual observation that the egg 

 does not at first contain any formed embryo whatever ; that the struct- 

 ure is wholly different from that of the adult ; that development is not 

 a mere process of unfolding, but a progressive process, involving the 

 continual formation, one after another, of new parts, previously non- 

 existent as such. This is somewhat as Harvey, himself following 

 Aristotle, had conceived it — a process of epigoicsis 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 egg and the mechanism of inheri- 

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

 for Schwann (1839) ^I'ld his immediate followers to recognize the 

 fact, conclusively demonstrated by all later researches, that the ej^x 

 is a eel! having the same essential structure as other cells of the 



'^Evolution and Epigenesis, Wood's HoU Biological Lectures, 1894. 



