ZOOLOGICAL SCIENCE 193 



advance from an observational study of structural changes 

 to an experimental analysis of causation. 



To follow another illustration: one of the most famous 

 disputes among the earlier embryologists was that of pre- 

 formation versus epigenesis. Is the organism already formed 

 within the germ, like the bud of a plant, and does develop- 

 ment consist merely in an unfolding of what is already exist- 

 ent ; or is development the coming into being of one feature 

 after another from a beginning that is without form and 

 void, in so far as any resemblance to the completed organism 

 is concerned? The preformationists of the eighteenth century 

 went so far as to develop an elaborate theory of encasement, 

 by which the germ was supposed to contain all of the adult 

 structures in miniature, including the germs of all future 

 generations, enclosed one after another in ever-decreasing 

 magnitude, like toy eggs within eggs carried inward to 

 infinity. Thus, the ovary of Eve could be supposed to have 

 contained the encapsuled representatives of all future gen- 

 erations of the human race. 



Of course, no very extensive knowledge of embryonic 

 stages was needed to demonstrate that the general course 

 of development in all animals is by epigenesis and not by 

 preformation. The fertilized egg possesses at the outset 

 no obvious resemblance to the future adult. Adult organ- 

 ization is attained through growth and cell division and by 

 gradual differentiation of parts (Fig. 12). Seemingly, there 

 could be nothing farther from an unfolding of what is al- 

 ready preformed. But the fact that two eggs, placed side 

 by side in a dish of water, develop into a frog and a toad, 

 or into a starfish and a sea-urchin, is evidence that some sort 

 of preformation does exist, unless one regards the develop- 

 ment of every individual as a supernatural process which 

 cannot be subjected to scientific analysis. Hence, the ques- 

 tion of epigenesis as opposed to preformation has engaged the 

 attention of experimental embryologists during recent years. 



The eggs of many animals, among others the frog, sea- 



