PLATHELMIM'HES 161 



After the four blastomeres have arranged themselves in 

 one plane, a small cell buds forth at the upper [aboral] part 

 of each cell. In this manner four cells arise, from which 

 subsequently the entire ectodei'm takes its origin (Fig. 75 B). 

 As soon as the four primitive ectoderm cells have come close 

 together, again four cells, the primitive mesoderm cells, are 

 budded off at the aboral pole of the large blastomeres. These 

 cells lie in such a position that thej are not covered by the 

 ectoderm cells (Fig. 75 G). The ectoderm cells then increase 

 to the number of eight. Four additional mesoderm cells have 

 meantime been constricted off from the large blastomex-es, 

 and the four already present have divided into eight. Ecto- 

 derm and mesoderm in the form of a cap overlie the four 

 large blastomeres, which from now on must be considered 

 as entodei-m (Fig. 75 D). At the lower pole of these four 

 primitive entoderm cells four smaller entoderm cells are 

 constricted off, a process which is repeated, and in the same 

 manner, at the upper pole (Fig. 75 E). We will state at this 

 point that it is the upper and lower entoderm cells which 

 supply the intestinal epithelium, whereas the large middle 

 ones constitute a kind of food-yolk and soon disintegrate 

 (Fig. 76 A and B). Even before the division of the primi- 

 tive entoderm cells has taken place, the cells of the ecto- 

 derm have considerably increased in number. They move 

 downward and begin to grow over the mesoderm cells. 

 Fig. 75 JB and F show these conditions in a diagi-ammatic 

 way. The further growth of the ectoderm now proceeds 

 rapidly, and the entoderm and mesoderm are soon entirely 

 covered by it. The formation of the epibolic gastrula is 

 herewith completed. The ectoderm becomes covered with a 

 dense coat of short cilia, and the embryo begins to rotate in 

 the egg-shell. 



We have represented the cleavage as Lang figures it for Liscocelis 

 tigrina. Although differmg in details, it still agrees on the whole with 

 the processes as they have been described for other Polijclads {Lcpio- 

 plana, Eurylepta) by Hallez and Selenka. The differences relate to the 

 formation of the mesoderm and entoderm. As regards the former, four 

 mesoderm cells are constricted off only once from the large blastomeres ; 

 and these by division then give rise to the mesoderm. According to 

 K. H. E. M 



