332 PROCEEDINGS OF THE ACADEMY OF [Julv, 



plane, but they later shift position so that the plane of division between 

 them comes to coincide with the plane of bilateral symmetry. The 

 next division of these cells separates an anterior protoplasmic portion 

 from a posterior yolk-laden one (fig. 9), and this process is repeated at 

 a slightly later stage (fig. 12), thus giving rise to six cells derived from 

 the cell 4rf, three on each side of the median plane. 



Of these cells the two anterior ones on each side are small proto- 

 plasmic cells which lie wholly under the layer of ectodermal cells; 

 these cells give rise to the mesoblastic band of each side. The posterior 

 cells are nmch larger and contain a consideral^le quantity of yolk; they 

 are only partially covered by the cap of ectodermal cells (fig. 12); 

 they ultimately give rise to the terminal portion of the intestine. I 

 have not followed in detail the cell lineage of these cells in later stages, 

 owing in part to the fact that they lie close under the thin layer of 

 ectoderm, and it is frequently difficult to distinguish one from the other 

 in surface views. However it is sufficiently evident that these cells 

 show fundamental resemblances in origin, history and destin}^ to the 

 mesentoblast cells of Crepidula ^nd other gasteropods. The "meso- 

 blastic bands" of Fulgur do not present a single series of cells, as in 

 some annelids and mollusks, but rather a broad, irregular l^and of 

 cells which has the general form of the letter Y. The stem of the Y 

 lies in the median plane behind the area of the shell gland, while its 

 two branches diverge on each side of this structure (figs. 19-24). 



Some distance in front of the anterior ends of these bands a few cells 

 appear beneath the ectoderm, which are probablj^ the homologues of 

 the "larval mesoderm" of other forms (figs. 19-22). They are in 

 close relation to the ectoderm cells which give rise to the cerebral 

 ganglia. I have not followed these "larval mesoderm" cells imtil 

 they give rise to mesodermal structures, but have classed them as 

 mesoderm because they lie beneath the surface and resemble in appear- 

 ance the cells of the mesodermal bands. 



6. THE ENTOMERES. 



Up to a stage when there are approximately fifty cells in the entire 

 embryo the entomeres consist of the four macromeres only. These 

 macromeres are all of about the same size, and they contain an enormous 

 quantity of yolk as compared with the amount of protoplasm. Each 

 macromere contains an area of cytoplasm relatively free from j^olk, in 

 which lie the nucleus and sphere. These cytoplasmic areas lie as near 

 as possible to the animal pole and to the free surface of the cell ; with 

 the extension of the cap of ectomeres these cytoplasmic areas with 



