228 



EMBRYOLOGY 



split ofP directly from tlie larval skin. This process takes 

 place on the free-swimming larva, for long before this the 

 embryo had broken through the egg- 

 membrane. Even at the time of its be- 

 coming free, it exhibits at its anterior 

 and posterior ends stout cilia (Fig. 108), 

 which are likewise a reminiscence of the 

 pilidium stage. 



Stout cilia, or tufts of cilia, arise at 

 the ends of the body of the embryo even 

 in those forms in which the development 

 has become quite direct {AmphiporuSj 

 Tetrastemma, Malacohdella) , and in which 

 no other points in harmony with indirect 

 development are still found, — apparently 

 an indication that development by 

 means of a ciliated free-swimming larva 

 Fi«t. 108.— Embryo of is the more primitive mode, direct de- 

 Cephaiothrix gaiathem yclopment, on the Contrary, the derived 



just hatched (after r ) j > 



DiBCKj. method. 



Cleavage and the production of the germ-layers in the forms developing 

 directly do not appear always to take place in the manner which we have 

 hitherto considered. In 3Ionopora vivipara, it is true, after an irregular 

 cleavage there arises from the hlastula an invagination gastrula (Salen- 

 sky) ; other forms, on the contrary {Amphiporus lactifloreus, Folia car- 

 cinophila, Tetrastemma varicolor), are said to possess a delamination 

 gastrula (Barrois, Hoff^iann). The sheet of long prismatic cells which 

 forms the hlastula splits into an outer and an inner layer. The former 

 corresponds to the ectoderm, whereas the latter again separates into a 

 double layer, the outer one the mesoderm, the inner one the entoderm. 

 In Tetrastemma the diJEferentiation of these cell-layers takes place in a 

 solid mass of cells, a part of which remains at the centre and is employed 

 only as food material. In Malacohdella also the germ is said to consist 

 of a solid cell-mass, from which the ectoderm becomes detached. Indi- 

 vidual cells migrate from the inner cell-mass into the cavity thus pro- 

 duced, and constitute the middle germ-layer. The remaining cells 

 correspond to the entoderm, and finally arrange themselves into an 

 intestinal epithelium, which fuses with the ectoderm to form the mouth 

 and anus. The embryonic development is now completed. The ciliated 

 embryo reaches the outside world to develop directly into the Nemertean 

 (Hoffmann). 



