PLATHELMINTHES 



179 



ectodenn, and which he believes has arisen, like the envelop- 

 ing membi'ane, as the result of an overgrowth, coming from 

 one side and surrounding the cell-mass. This, then, would 

 be an epibolic gastrula (Fig. 87 E andi^). 



The further changes of the embryo consist, in the first 

 place, in the gradual disappearance of the nuclei of the 

 ectoderm cells and transformation of the entire ectoderm into 



PcG. 8".— .4 to H, embryonic development of Bistomum tereticolle (after Schauin^- 

 land). D, intestine ; Dz, yolk-cells ; Ez, egg-cell ; Ec, ectoderm ; En, entoderm ; Urn, 

 enveloping membrane ; Kz, cap-cell. 



a thin cuticula-like layer, on the surface of Avhich bristle- 

 like structures make their appearance (Fig. 87 H). A 

 number of the cells of the entoderm have united for the 

 formation of the intestine, which fills about one half of the 

 body (Fig. 87 G). Other entoderm cells are applied to the 

 ectodermal membrane, whereas the remaining cells, lying 

 between these and the intestine, retain the character of 

 embrj'-onic cells. They are germ cells, from which the new 

 generation subsequently arises. Since in the present stage 

 the cells of the ectoderm, as well as those which form the 



