PLATHELMINTHES 



191 



a relatively small amount of yolk material. The embryos 

 contained in them are naked. 



The embryonic development of the Bothriocepha- 

 lidae approaches closely that of the Distomida3. Cleavage 

 takes place in much the same way as there. At an early period 

 two cells are differentiated at the two poles of the elongated 

 germ, upon which they rest like a cap. They then grow 

 around it, and constitute the enveloping membrane (Hiill- 

 membran). Afterwards another cell is separated off from 

 the spherical cell-mass sui-rounded by the enveloping mem- 

 brane, and this at first also covers the germ like a cap, and 

 then grows around it. Later this 

 external layer consists of several 

 cells. It is in this way that the 

 ectoderm is formed. The embryo 

 now consists of a single layer of 

 ectoderm and a solid entodermal 

 mass (Fig. 95). Sis chitinous 



hooks make their appearance in 

 the latter. With this the forma- 

 tion of the embryo is completed. 



It is said to be composed of the 



inner (entodermal) mass only. 



The ectoderm separates from it, 



so that a space arises between 



the two. The embryo is now 



surrounded by two envelopes in 



addition to the egg-membrane, 



the ectodermal mantle, and the 



enveloping membrane. In this 



respect, too, the conditions described for the Distoraida? are 



repeated, and a comparison of Fig. 95 with Fig. 88 (on p. 



180) shows without farther comment the close agreement of 



the two groups at this stage of development. 



Whereas the embryo quitting the egg leaves the enveloping 



membrane behind in the egg-shell, it takes the ectodermal 



mantle with it (Fig. 95). The latter either serves actively 



in locomotion when it possesses cilia, or it swells up so much 



in the water that it serves the larva both as a protective 



Fig. 95. — Embryn of BotKrioce- 

 phalus latus pressed out of the egg. 

 Ec, ect yderm ; Hin, enveloping 

 membrane (after ScHAUiNSLiNo). 



