PLATHELMINTHES 



191 



a relatively small amount of yolk material. The embryos 

 contained in them are naked. 



The embryonic development of the Bothriocepha- 

 lldaB approaches closely that of the Distomidse. 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 (Hilll- 

 tnembran) . Afterwards another cell is separated off from 

 the spherical cell-mass surrounded 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). Six 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 Distoraidse are 

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

 180) shows without further 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,— Embrj'O of BotKrioce- 

 phalus latus pressed out of the egg. 

 Ec, ectjderm ; Km, enveloping 

 membrane (after ScHA.TJiifSLA.ND). 



