Localization of the N emertine Egg. 327 



While the larvae developed from Isolated blastomeres of the 

 two and four-celled stage show certain organic differences from 

 the normal whole larvse, there is no indication in them of a specific 

 local defect. The first trace of such a defect is reached in the 

 eight-cell stage. Here, while larvse developed from lateral four- 

 cell groups containing two cells of the upper quartet and two cells 

 of the lower show the characters of a normal larva (except for 

 asymmetry in arrangement) larvae from the upper quartet al- 

 ways possess an apical organ and lack an enteron, and those from 

 the lower possess an enteron and lack an apical organ. There is 

 thus a very distinct differentiation along the apical-basal (polar) 

 axis. It is an interesting fact that this first distinctive morpho- 

 genic localization is coincident with the first inequality in cleavage, 

 the Inequality being In the same direction. Projecting backward 

 this differentiation — that Is, assuming that an apical-basal differ- 

 entiation has been going on for some time before the eight-cell 

 stage — naturally there would be no indication of It In the Isolated 

 blastomeres of the two or four-cell stages because the cleavages 

 are vertical. Likewise there may be a similar differentiation in 

 the unsegmented egg; for, while my results on cleavage defects 

 cannot be analyzed as showing any specific relation to the indi- 

 vidual kinds of egg defects, the observations of Yatsu on morpho- 

 genlc defects do show such a relation. 



The experiments on the eggs of Cerebratuhis marginatits, to- 

 gether with the former ones on C. lacteus, seem therefore to indi- 

 cate that at the eight-cell stage the formative materials of the 

 egg are definitely localized in an apical-basal direction, and the 

 experiments of Yatsu on morphogenic defects In larvae resulting 

 from unsegmented eggs of the later maturation stages show a 

 similar apical-basal differentiation. That this process of apical- 

 basal differentiation is a progressive one In the unsegmented egg 

 Is Indicated by the whole character of the cleavage in fragments 

 of unfertilized eggs, and by the progressive departure from this 

 character up to the first cleavage, and the corresponding increase 

 In defects of larvae developed from the fragments, though In the 

 latter case the continuity of the result seems to be masked by the 

 development of whole larvae from isolated one-half and one-fourth 



