PREFORMATION AND EPIGENESIS 437 



the encapsuled representatives of all future generations 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 fer- 

 tilized egg possesses at the outset no obvious resemblance to the 

 fully developed individual. The adult organization is attained 

 through growth, by cell division and gradual differentiation {cj. 

 Fig. 213, p. 405). Seemingly, nothing could be farther from an 

 unfolding of what has already been 'preformed. But the fact that of 

 two eggs, placed side by side in a dish of water, one develops into 

 a frog and the other into a toad, or one into a starfish and the 

 other into a sea urchin, is evidence that some sort of preformation 

 does exist, unless one regards development as a supernatural 

 process which cannot be subjected to scientific analysis. Hence, 

 the question of epigenesis as opposed to preformation remains, in a 

 modified form, as a problem for the embryologist of the present 

 day. This problem has been attacked experimentally in animals 

 like the frog, sea urchin, and starfish, the eggs of which are fertilized 

 and develop in the external water. In such instances one is able 

 to make experiments that cannot be made upon eggs developing 

 within a brood pouch or other internal cavity of the parent, and 

 it is possible to determine whether the protoplasm of the egg is 

 preformed, to such an extent that certain of its parts can only give 

 rise to certain parts of the adult, or whether there is more latitude 

 in the developmental processes. For example, pieces may be cut 

 from different regions of the fertilized or unfertilized egg of the 

 sea urchin, and the four, eight, and even sixteen-cell stages may be 

 separated into their component cells. These and many other 

 experiments can be performed for the purpose of demonstrating 

 the nature of the organization that the egg may possess, since it is 

 obviously something within the egg that determines the major 

 features of development. 



So many and so diversified have been these experiments that 

 only the general results can be mentioned. The eggs of many 

 animals exhibit within their cytoplasm {cf. Fig. 112, p. 224) 

 substances, unlike the adult parts but marking the places from 

 which these parts originate. Such eggs are organized or -preformed, 

 to the extent that certain regions of the egg become definite 

 regions of the adult. The eggs of other animals exhibit little 



