244 The American Naturalist. [March, 



from the central protoplasm a network extends out through the yolk 

 to a peripheral layer. It is possible to determine the second plane of 

 division as that of the long axis of the embryo as has been shown to be 

 the case in Nereis, Crepidula, and Umbrella. In discussing the 

 segmentation, the author uses the term cell as a convenient one for the 

 nucleated bodies of protoplasm which appear on division in the yolk, 

 but he insists on the syncitial nature of the ovum to a late period. 



The cleavage is apparently a spiral one, and results in what may be 

 spoken of as a blastula stage, in which considerable differentiation has 

 taken place. From the eight cell stage, especially in Jjera, it is possible 

 to trace the history of different areas of this blastula to certain well 

 marked cells. For instance, in Jaera, a cell at the posterior pole 

 gives rise to the future Vitellophags, three cells immediately en- 

 circling it are the ancestors of the mesendoderm, while the ectoderm 

 arises from the remaining four anterior cells. There are some interest- 

 ing variations in this history in the forms studied, though the end result 

 is practically the same. The author concludes with E. B. Wilson that, 

 " cells having precisely the same origin in the cleavage, occupying the 

 same position in the embryo, and placed under the same mechanical 

 conditions may nevertheless differ fundamentally in morphological 

 significance." 



In connection with the segmentation Dr. McMurrich thinks that the 

 existence of a syncitium up to so late a period in differentiation is of 

 special interest in relation to the current disscusion of the cell-theory. 

 The question is asked, " are we to believe that there is no continuity in 

 Lucifer, between the blastomeres, notwithstanding that in all prob- 

 ability there was continuity in the ova of its ancestors ? " In Peripatus 

 capensis there is an approach to holoblastic cleavage associated with 

 less yolk and still a syncitium results. This is regarded as supporting 

 " the supposition that, even in such cases as Lucifer, there may be also 

 a continuity of protoplasm, the separation into distinct spherules being 

 only apparent." 



It should be remembered that, however, plausible this argument is, of 

 course the fact of continuity between the blastomeres of Lucifer or of 

 other holoblastic ova still remains to be proven by direct observation. 



The conclusion that " the existence of a syncitium is no bar to a cer- 

 tain amount of differentiation," certainly seems justified from the facts 

 described for Jsera. Continuing this subject, such a syncitium is com- 

 pared with the differentiation in certain protozoa, and a peculiar 

 phenomenon in Porcellio is considered, where there i 

 segregation of a portion of the cytopla 



