108 



EMBRYOLOGY. 



separate the intestine from the wall of the body. They appropri- 

 ately take the name enter ocod, since they are formed from the coelen- 

 teron by a process of constriction, and are genetically distinguishable 

 from other cavities which arise in other animals between the wall of 

 the intestine and that of the body by simple splitting, and to which 

 is given the ii&m.e jissicoel or schizoccd. 



By the jyrocess of infolding the number of the germ-layers in Sagitta 

 has been increased from two to three. The primary inner germ-layer 

 is thereby divided into (1) a cell-layer (ik) which lines the intestinal 

 tube, and (2) a cell-layer which serves to enclose the two body-cavities 

 (mk l and mk 2 ). The first is designated as the secondary inner germ- 

 layer or entoblast, the second as the middle germ-layer (mesoblast). 



One part of the latter is adjacent to the 

 outer germ-layer, the other part to the 

 intestinal tube ; accordingly the division 

 is carried still further into a parietal 

 (mk 1 ) and a visceral layer (mk 2 ) of the meso- 

 blast. For the sake of brevity the former 

 may be called the parietal (mk 1 ), the latter 

 the visceral (mk 2 ) middle layer. Conse- 

 quently, one may now speak of two middle 

 germ-layers instead of one, the total number 

 of the germ-layers being, naturally, raised 

 by this from three to jour. 



Fig. 67. Diagrammatic cross sec- 

 tion through a young Sagitta. 



dM, Dorsal, vM, ventral mesen- 

 tery ; dh, intestinal cavity ; 

 Ih, body-cavity ; ak, outer, ik, 

 inner germ-layer; wife 1 , "parietal, 

 mk", visceral middle layer (mid- 

 dle germ-layers). 



In regard to the course of the further 



development it may be stated that, while 

 the larva elongates into a worm-like body, 

 the two body-sacs (fig. 67 Ih) are increased to a greater extent 

 than the intestinal tube (ah) which they embrace. They everywhere 

 crowd the latter away from the wall of the body, grow around it 

 from above and below, where their thin walls come into direct con- 

 tact. By the fusion of the two body-sacs along their surfaces of 

 contact there are formed two delicate membranes, a dorsal (dM) 

 and a ventral (vlf) mesentery, by means of which the intestinal tube 

 is attached to the dorsal wall and to the ventral wall of the 

 trunk. 



Processes very similar to those of Sagitta occur in the development 

 of Vertebrata also, but in the latter case they are combined with 

 the development of the neural tube and the chorda dorsalis. In the 

 presentation of these we shall proceed as in the foregoing chapter, 

 which treated of the formation of the gastrula, and consider separately 



