162 



EMBRYOLOGY 



Selenka, entoderm cells are formed at the lower pole only of the large 

 blastomeres. According to Goette (in SUjlochus), such a differentiation 

 of the mesoderm as is described by other authors does not take place. 

 The cells, which in his figures seem to correspond to the mesoderm cells, 

 he considers as ectoderm. According to Goette, Stylochue, in which 

 the development of a mesoderm has not yet taken place, represents a 



Fig. 75.— 4 to F, cleavage stages of tV,e eggs of Polyclads (after Lang). A, dia- 

 gram of a stage of four blastomeres, of which the two larger ones correspond to 

 the anterior (u) and posterior (/«) parts of the body, the smaller ones, lying above 

 them, to the sides of the body; i^to D, more advanced stages of Biscocelis tigrina, 

 B and C seen from above, D from the side ; JP, later stage of the epibolic 

 gastrula of T?ij/sn»i02o6?i Brochii, seen from the side. Ec, ectoderm; En, ento- 

 derm ; 0. En and k. En, upper and lower entoderm ; Me$, mesoderm. 



more primitive condition than the rest of the Polycladida. This con- 

 dition follows as a result of the shape of the entoderm. The central 

 entoderm cells do not in this case become food-substance, but with the 

 others form the wall of the intestine (Fig. 76 C). Everything enclosed 

 by ectoderm Goette looks upon as entoderm. Only after a part of the 



