84 CHILD. [VOL. I. 







determined, as Hertwig states ('97c) by the arrangement of the 

 yolk. As was pointed out above, the yolk, though distributed 

 throughout the unsegmented egg, may go to all the cells, and 

 yet these cells may exhibit great differences in size, or it may 

 appear in only a small number of them. Both the form of 

 cleavage and the distribution of the egg-substance are deter- 

 mined by some power in the egg, which acts, at least in part, 

 according to laws which are not yet understood. This power 

 constitutes the true organization of the egg. The fertilized 

 egg is not divided into pigeon-holes containing substances for 

 different portions of the adult body, but it is simply a cell pos- 

 sessing the power to initiate certain chemical and physical 

 processes, which in their turn initiate others, until, as the final 

 result, the adult appears. This is epigenesis pure and simple, 

 but it differs from Hertwig's position in that it recognizes 

 a more fundamental organization in the egg than the visible 

 one consisting of protoplasm and cleutoplasm. 



This organization is in general terms the specific nature of 

 the reproductive cell. It is a phase of the same power that 

 determines that the egg of Arenicola cristata shall develop into 

 Arenicola cristata. At present we are in the dark concerning 

 it. According to Weismann it is the germ-plasm, but what- 

 ever it be called, it is present from the beginning, and the 

 visible cytoplasmic organization, the form of cleavage, and 

 the whole ontogeny are the result of it. 



* 

 ZOOLOGICAL LABORATORY, 



UNIVERSITY OF CHICAGO, 

 April, 1897. 



