No. I.] THE EMBRYOLOGY OF CREPIDULA. 75 



meres divide more rapidly than in others. But in all these 

 cases the cell 4d is formed in the fourth quartette of cells sepa- 

 rated from the macromeres. The essential likeness in origin 

 of this cell in all these forms is thus clearly shown, though it 

 arises at apparently different tiro.es in different eggs. 



In addition to the mesoblast thus formed, which is bilateral 

 and teloblastic in growth, three other mesoblast cells arise 

 from the ectoblast in Crepidula at a much later stage. These 

 cells, which correspond to the "larval mesoblast" of Unio 

 (Lillie ('93), p. 570), appear in the quadrants A, C, and B, and 

 give rise to the scattered mesoblast cells in the region of the 

 blastopore, and at the anterior end of the embryo. The origin 

 of these cells cannot be described satisfactorily until the later 

 history of the ectoblast has been considered (see p. 149). 



Although I do not propose in this section to take up the his- 

 tory of the different layers, yet it seems best here to describe 

 the complete separation of the fourth quartette of cells to 

 which 4d belongs. 



4. Completion of the Fourth Quartette and Rotation of the 

 Ectoblast. Figs. JJ, J^. 



A laeotropic division in the 24-cell stage separated the 

 mesentoblast from the left posterior macromere ; the corre- 

 sponding divisions in the other macromeres are delayed until 

 the stage with 49 cells. Figs. 33, 34. At this stage each 

 of the macromeres, except the left posterior one, gives rise to 

 a large yolk cell, 4a, 4b, and 4c, by a laeotropic division. The 

 cells thus formed are a little larger and contain much more 

 yolk than the mesentoblast 4d. They move around into the 

 furrows between the macromeres, and ultimately take part in 

 forming the ventral wall of the mesenteron. 



No divisions corresponding to this are given for Nereis or 

 Unio.^ In Limax and Planorbis the cells 4a, 4b, 4c are sepa- 

 rated at the same time with 4d; in Umbrella at about the same 

 stage as in Crepidula; in Neritina two cells which seem to cor- 



1 Mead ('94) briefly mentions the fact that these divisions occur in Amphitrite, 

 Clymenella, and Lepidonotus. 



