194 THE SCIENCE OF BIOLOGY 



urchin, and starfish, are fertilized and develop in external 

 water without parental care. Here, experiments are pos- 

 sible which could hardly be made upon an egg developing 

 within a brood-pouch or other internal cavity of a parent. 

 The question of whether the protoplasm of the egg is pre- 

 formed, to the extent that certain of its parts are destined 

 to give rise to certain parts of the adult, has evoked con- 

 siderable interest. The problem has been attacked experi- 

 mentally by the removal of parts of the egg in the sea- 

 urchin and other forms. Pieces have been cut from different 

 regions of the fertilized and the unfertilized egg; two, four, 

 eight, and even sixteen cell stages have been separated into 

 their component cells. These and many other experiments 

 have been performed, with a view of demonstrating the 

 nature of the organization, which must be postulated, 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 we can summarize only their general outcome. The 

 eggs of many animals exhibit within their cytoplasm (Fig. 

 12) recognizable substances, unlike the adult parts but 

 from which the adult parts take origin. Such eggs are thus 

 visibly organized or preformed to the extent that certain 

 regions of the egg become certain regions of the adult. The 

 eggs of other animals exhibit little differentiation which can 

 be, at present, recognized. In eggs of the latter sort, one 

 area is more nearly of the same value as every other, and 

 recognizable differentiation appears at a subsequent stage 

 of development. The truth seems to be that the eggs of 

 different animals are not alike w T ith respect to their visible 

 differentiation at the one cell stage; that the first signs of 

 differentiation, while visible in some animals at the one cell 

 stage, are less apparent in others until a later stage of develop- 

 ment; while in those forms which have as adults great 

 capacity for the regeneration of lost parts, the organism 

 is never so completely differentiated as to be unable to re- 



