DEVELOPMENT OP THE TWO MIDDLE GEEM-LAYEES. 



131 



A similar view is furnished by a cross section through the cephalic 

 process of the germ of the Chick (fig. 93). Under the outer germ- 

 layer there is found in the median plane, in front of the primitive 

 groove, only the fundament of the chorda (cA) ; at the point indicated 

 by a star it is continued laterally into the small-celled middle germ- 

 layer, and into the entoderm, which is composed of a single layer of 

 very much flattened cells. 



The same is true for cross sections of Mammals (fig. 94) in corre- 

 sponding stages of development. Thus, for example, the funda- 

 ment of the chorda {cK) in the cross section through the embryo of a 

 Mole figured by Heape is a single layer of cylindrical cells ; it has 

 ah'eady become curved into a chordal groove, such as has been repre- 

 sented in fig. 79 .4 for Triton. Laterally it is continuous with a 

 mass of small cells, which is resolved into two layers at the point 



Fig. 94. — Cross section through the embryonic area of a Blole which is in ahout the stage of th& 



Rabbit represented in Fig. 89 B. After Heafe. 

 The section passes through the chordal gi'oove (cA) somewhat farther forward than the section 



representbj in Fig. 97, wliich has encountered a region that is to be interpreted as the 



blastopore. 

 ale. Outer, mk, middle, ih, inner germ-layev ; eh, fundament of the chorda. 



indicated by a star : (1) into the middle germ-layer {mh), composed 

 of several layers of small cells ; and (2) into the inner germ-layer, 

 which, as before, appears as a single layer of flattened cells (iA). 



In a still more convincing manner van Beneden has shown, in his 

 investigations upon the development of Mammals, that conditions 

 exist in the formation of the middle germ-layer and of the body- 

 cavity in this class which agree with those in Amphibia. The cross 

 section (fig. 95) through the germ-disc of the Eabbit, taken from 

 his work, is especially convincing. It shows the fundament of the 

 chorda (cA) as a single layer of cylindrical cells, flanked on the right 

 and left by the middle and inner germ-layers. The middle germ- 

 layer consists of a parietal (mh^) and a visceral (mW) lamella of flat 

 cells, the former of which is continuous with the fundament of the 

 chorda, whUe the latter bends around at the point indicated by a 

 star to become continuous with the single-layered epithelium of the 



