114 



EMBRYOLOGY. 



the yolk-mass and the outer germ-layer. Their wall (ink 1 and mk a ) 

 is composed of small cubical or polygonal elements, shaded darker 

 in the diagram. The ccelenteron is distinctly separated by means 

 of the two coelenteric folds (* *) into a median or intestinal cavity 

 proper (dh), lying beneath the chordal fundament, and the two narrow 

 body-sacs (Hi), which communicate with the former only by means 

 of narrow fissures (* *) at the right and left of the chordal funda- 

 ment. The figure is easily reducible to the preceding (p. 113) cross 

 section of an Amphioxus embryo (fig. 74), if we conceive the simple 

 epithelium on the ventral side of the latter thickened by an accumula- 

 tion of yolk, and the two 

 small body-sacs grown 

 down a certain distance 

 between yolk-mass and 

 outer germ-layer. 



In the second dia- 

 grammatic cross section, 

 which is through the 

 blastopore (fig. 75), the 

 coelenteron (ud) is wholl} 

 filled up with the yolk 

 mass (d). The body-sac^ 

 (Hi) described in the first 

 diagram are to be seen 

 here also, as they crowd 

 themselves downwards 

 between yolk and outer 

 germ-layer. Their walls 



are composed of small cells, and the outer or parietal layer (mk l ) 

 merges into the outer germ-layer at the blastopore, while the inner 

 or visceral layer (m& 2 ) is continuous with the yolk-mass or the inner 

 germ -layer. 



Were the conditions in Vertebrates such as the two diagrams 

 represent, there could no longer be any doubt in regard to them, 

 any more than in the case of Amphioxus, that the body-cavity is 

 developed out of two evaginations of the coelenteron, and that its 

 Avails constitute the two middle germ-layers. But there is not a 

 single Vertebrate which presents such clear and convincing evidence. 

 The distinctness is everywhere diminished, most of all by the 

 fact that the parts which are to be interpreted as body-sacs no longer 

 enclose cavities, because their walls are firmly pressed together, in 



Fig. 75. Diagram to show the development of the middle 

 germ-layers and the body-cavity in Vertebrata. 



Cross section through the blastopore of an embryo. 



u, Blastopore ; ud, coelenteron ; Ih, body-cavity ; d, yolk ; 

 ak, outer germ-layer ; mk\ parietal, ml?, visceral 

 lamella of the middle germ-layer. 



