No. 2.] EARLY DEVELOPMENT OF PLANORBIS. 413 



lids and mollusks with that of the polyclades, in which the 

 mesoderm has a radial origin from one or more quartettes of 

 micromeres. Until more is known, however, of the origin, and 

 especially of the fate, of the cells which have been called sec- 

 ondary mesoblasts, it must remain uncertain whether there is 

 any true homology between these cells and the mesoblast of 

 the polyclades, although such a comparison naturally suggests 

 itself. The subject is one of considerable interest from the 

 standpoint of phylogeny, and the reader may be referred for 

 suggestive discussions of the problem to the papers of Conklin 

 {'97), Wilson ('98), and Eisig ('98). 



The Entovieres. 



The Q.gg of Planorbis is peculiar among the eggs of mollusks, 

 in that the entomeres are of small size and undergo numerous 

 divisions before the beginning of invagination. The fourth 

 quartette consists of cells which greatly exceed in size the four 

 small cells at the vegetal pole. The three cells of this quartette 

 which form entoderm, ^a, 4b, and 4^, divide horizontally when 

 the egg contains about fifty-six cells. The six cells resulting from 

 this cleavage are arranged in the form of a horseshoe about the 

 four cells in the center, the opening between the ends of the 

 curve being on the posterior side of the egg. The next cleav- 

 age occurs when the egg contains about ninety cells. Each of 

 the six cells of the fourth quartette divides in a nearly radial 

 direction. These divisions are, however, slightly oblique, and 

 are bilaterally symmetrical with respect to the median plane of 

 the egg. The divisions of the cells on the right side of the Qgg 

 are slightly laeotropic, while those on the left side are slightly 

 dexiotropic. These divisions are followed by a cleavage of the 

 three small cells. A, B, and C, at the vegetal pole, forming a 

 fifth quartette. The cell D, which is the smallest of the group, 

 does not divide ; nor have I been able to observe its cleavage at 

 any subsequent stage, although it could be recognized after the 

 process of gastrulation had made some progress. It is quite an 

 exceptional fact that the cell D, which, in many forms, is the 

 largest cell of the egg, should, in Planorbis, be the least in size 



