DIFFERENTIATION, HEREDITY, SEX 287 



That the " organization " of the cytoplasm results from a 

 progressive developmental process is clearly evidenced by the 

 experiments of Wilson, Yatsu, and Zeleny on the egg of 

 Cerebratulus before cleavage. If portions of this egg are 

 removed before maturation has begun, while the egg nucleus 

 is still in the form of an intact germinal vesicle, no defects are 

 seen in the resulting larva. Entrance of the 'spermatozoon is 

 followed by maturation and a general rearrangement of the 

 substances of the cytoplasm, one result of which is the forma- 

 tion of a cap of clear protoplasm at the animal pole. Removal 

 of this substance prior to or during the first cleavage, often pro- 

 duces no later abnormality. The separated blastomeres of the 

 two-cell stage, however, while for a time cleaving like halves, 

 soon assume the character of wholes. Those of the four-cell 

 stage continue longer to behave like parts, even through the 

 blastula stage, although ultimately they may form typical 

 free-swimming larvaB. The degree of defect corresponds in a 

 general way with the stage to which cleavage has progressed 

 at the time of separation. Larvae developed from eggs with- 

 out the upper quartet, which contains the clear protoplasm 

 mentioned, have typically formed enteron, but lack the apical 

 organ. Larvae from this upper quartet have the apical organ 

 but are without enteron. And the same is true when, in the 

 sixteen-cell stage, the upper and lower octets develop sep- 

 arately. Parts of the blastula continue to develop for a time 

 and form only the restricted cell groups to which they give rise 

 in normal development. 



Such facts seem clearly to mean that cytoplasmic germinal 

 localization may be complete in later stages, but incomplete or 

 absent in the earlier, that it is truly a process or result of devel- 

 opment and not a primary determiner of the course of develop- 

 ment, not a fixed thing persisting from generation to generation, 

 which might be regarded as the physical basis of heredity. 



The conception of cytoplasmic localization as a progressive 

 process, i.e., as one factor or link in the chain of developmental 

 events, immediately raises the question as to what condition 



