PLATHELMIXTHES 161 ^ 



After the four blastomeres have arranged themselves in ''^^t 



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 ectoderm 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 oif at the aboral pole of the large blastomeres. These 1 



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



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



to the number of eight. Four additional mesoderm cells have -^ 

 meantime been constricted off from the large blastomeres, 

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

 derm and mesoderm in the form of a cap overlie the four c^ •^'2- - i ^ 

 large blastomeres, which from now on must be considered 



as entoderm (Fig. 75 D). At the lower pole of these four t 



primitive entoderm cells four smaller entoderm cells are j 



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



manner, at the upper pole (Fig. 75 -E7). We will state at this 1 



point that it is the upper and lower entoderm cells which j 



supply the intestinal epithelium, whereas the large middle i 



ones constitute a kind of food-yolk and soon disintegrate J 



(Fig. 76 A and B). Even before the division of the primi- 5 



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



derm have considerably increased in number. They move j 



downward and begin to grow over the mesoderm cells. i 



Fig. 75 E and F show these conditions in a diagrammatic i 



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 i 



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



the egg-shell. l 



We have represented the cleavage as Lang figures it for Liscocelis i 



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



the processes as they have been described for other Polyclads (Lepto- i 



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



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 



