No. 3.] POD ARK E OBSCURA VERRILL. 463 



an homology between the larval mesoblast of the annelid and 

 mollusk, and the adult mesenchyme of the platode. (See the 

 section on the mesoderm, p, 449.) 



Another difficulty in the way of cell homology has been the 

 cleavage of Polychoerus, which, according to Gardiner (No. 9), 

 is of the bilateral type. With the aid of Leptoplana and Disco- 

 coelis for comparison, Wilson ^ has reexamined Polychoerus, 

 and finds that it has the true spiral type of division and is thus 

 brought into line with the other genera. 



In the light of these facts Wilson has abandoned the position 

 taken in his lecture and has gone back to that of his first paper 

 on Nereis, though his views are even more extreme than in 1892. 

 His conclusions may be summarized in the following extracts 

 (No. 34, g) : " The phenomena shown in the history of the 

 micromere quartette in annelids, platodes, and mollusks render 

 it highly probable, if they do not actually demonstrate, that 

 development may exhibit ancestral reminiscence as clearly in 

 the cleavage of the ovum as in the later formation of tissues 

 and organs. These facts may well give us hope that when the 

 comparative study of cell lineage has been carried farther, the 

 study of the cleavage stages may prove as valuable a means for 

 the investigation of homologies and of animal relationships as 

 that of the embryonic and larval stages. 



"These facts seem on the whole to emphasize the impor- 

 tance of cell formation in development, and it would be difficult 

 to explain ancestral reminiscence {e.g., the small •' entero- 

 blasts " of Aricia, which he regards as ancestral rudiments, and 

 the larval mesenchyme of Unio or Crepidula] in cell lineage 

 under any view which does not recognize in cell outlines the 

 definite boundaries of differentiation areas in the developing 

 embryo." 



Eisig's study of Capitella cleavage brings to light many 

 similarities with the cleavage of other annelids, especially with 

 Nereis (No. 34, d), which he used most frequently for com- 

 parison, and he finds that in general the cleavages are remark- 

 ably alike in the two cases ; so that " Die Parallele, welche 



^ I am indebted to Professor Wilson's courtesy for permission to quote this, 

 as yet unpublished, observation. 



