652 



THE AMEBIC AN NATURALIST [Vol. LI 



distribution and that they might be qualitatively differ- 

 ent. If this were true, then nuclei of one sort might 

 always migrate into one part of the egg and might deter- 

 mine the nature of the tissue that developed there, and 

 nuclei of other sorts might likewise become located in 

 other predetermined parts of the egg. Careful studies of 

 the origin and migration of the cleavage nuclei^^ have 

 led to the conclusion that the distribution of these nuclei 

 is adventitious and that they are all potentially alike— 

 that is are totipotent— a view that is now held by most 

 embryologists regarding the relative importance of 

 nucleus and cytoplasm during cleavage. That the nuclei 

 may play a part in the ditferentiation of the cortical layer 

 of cytoplasm during the cleavage period is highly im- 

 probable, since definite cytoplasmic organization already 

 exists before cleavage begins. The factors brought into 

 the egg by the spermatozoon, however, have an oppor- 

 tunity at this time to modify the initial organization and 

 thus the early embryo may exhibit paternal characteris- 

 tics. Whether or not such an influence is exerted at this 

 time is not known. 



The kind of tissue that develops from any part of the 

 egg, therefore, depends upon the kind of cytoplasm en- 

 countered by the nuclei. 



{d) Complexity of Organization as Indicated hy the 

 Development of Parts of Eggs.— More convincing evi- 

 dence of the presence of a complex and fixed organiza- 

 tion in the cytoplasm was derived from operations per- 

 formed upon eggs with a hot needle. Parts of the freshly 

 deposited eggs were killed by being touched with a hot 

 needle ^nd these parts were thus prevented from taking 

 part in development. The living portions of the eggs 

 continued to develop and in every case produced those 

 parts of the embryo that they would have fbrmed if the 

 egg had not been injured.^*^ This seems to prove that 

 every part of the egg cytoplasm is set aside for the pro- 

 is Hegner, 1914, Jonrn. Morph., Vol. 25. 



