THE DISC-GASTEULA. 



219 



Fig. 4). The slightly arched floor of this low cleavage- 

 cavity is formed of nutritive yelk (n) ; the more arched roof 

 is of Blastula-cells. In fact, the embryonic Fish is now a 

 vesicle with an eccentric cavit}", as was the Blastula of the 

 Frog (Plate II. Fig. 10). 



The important process of inversion, resulting in gastru- 

 lation, now takes place. In consequence of a further re- 

 moval, or wandering, of the blastula-cells, and of a further 

 increase in their number, the thickened edges of the cellular 

 disc, which lie on the nutritive yelk, grow toward each 

 other in a centripetal direction, and toward the centre of 

 the cleavage-cavity (Fig. 23), at which point they finally 

 unite. The whole cell-mass now forms a small flat sac lying 

 on the top of the nutritive yelk. The cavity of this sac, 

 the cleavage-cavity, soon, however, disappears, because the 

 whole upper surface of the lower wall of the sac attaches 

 itself closely to the whole lower surface of the upper wall 

 (Fig. 24). This completes the gastrulation of this Fish. 



Fig. 43. — Disc-gastrula {Disco-gas- 

 triila) of an Osseous Fish : e, exoderm ; 

 i, entoderm ; tv, swollen edge, or primi- 

 tive mouth-edge ; n, albuminous ball 

 of nutritive yelk ; /, fat-globule with- 

 in the latter ; c, outer egg-membrane 

 (chorion) ; d, boundary between onto- 

 derm and exoderm (former site of the 

 cleavage - cavity) . 



In order to distinguish this third important form of 

 Gastrula from the two previously mentioned, we will call it 

 the Disc-gastrula (Disco-gastrula, Fig. 43). The cell-mass of 

 this Gastrula forms a thin, circular disc. The lower concave 



