No. 584] BILATERALITY IN VERTEBRATES 505 



including Morgan and Tsuda, Assheton, H. V. Wilson, 

 King, Smith and others have generally agreed that the 

 head end of the embryo forms from the active hemi- 

 sphere and the caudal portion from the inactive. My 

 own experiments on a considerable number of Amphibia 

 have led to the conclusion that the head of the embryo 

 forms from material which lies at, or near, the active pole 

 of the egg. It thus seems fair to assume that the cephalic 

 portion of the embryo is formed from the active hemi- 

 sphere. 



As stated there have been many attempts to deter- 

 mine how early in development the egg shows bilateral 

 symmetry. Some claim bilateralism for the primitive 

 ovum. Others hold that this condition is not present 

 from the first, but originates at some later period. This 

 period may precede or follow the deposition of the egg. 

 Those who regard the egg as bilaterally symmetrical 

 before deposition claim that this is manifested either 

 through an excentric position of the egg nucleus, or an 

 excentric pigmentation. Those who regard it as fixed 

 after deposition are not in accord. By some the path of 

 the spermatozoon is considered as the determining factor, 

 by others the first or second cleavage groove, and by still 

 others areas of accelerated segmentation. 



The assumption that the egg is bilaterally symmetrical 

 from the beginning is based upon nothing more than 

 plausible hypothesis and naturally falls beyond the range 

 of experimental proof. 



Some (Schultze) hold that the excentric position of the 

 egg nucleus together with the primary ovic axis deter- 

 mine bilaterality. The work by Roux, Jordan and others, 

 shows that this is highly improbable. 



Others (Roux, Morgan and Tsuda) maintain that the 

 excentric arrangement of pigment enables one to deter- 

 mine bilaterality. Professor Whitman's observations 

 which are recorded in a later paragraph, together with 

 his drawings, indicate that the arrangement of the pig- 

 ment is of significance in Bufo. The observations of 

 Moskowski on Bona, Morgan's later observations on 



