DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS. 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 (ch) ; 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 (ch) in the cross section through the embryo of a 

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

 already become curved into a chordal groove, such as has been repre- 

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

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



ak 



Fig. 94. Cross section through the embryonic area of a Mole which is in about the stage of the 



Rabbit represented in Fig. 89 B. After HEAPE. 

 The section patses through the chordal groove (ch) somewhat farther forward than the section 



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



blastopore. 

 ak, Outer, mk, middle, ik, inner germ-layer ; ch, fundament of the chorda. 



indicated by a star : (1) into the middle germ-layer (mk), 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 (ik). 



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 (ch) 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 (mk' 1 ) and a visceral (mk 2 ) lamella of flat 

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

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

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



