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 



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 passes through the chordal groove (c/i) somewhat farther forward than the section 



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



blastopore. 

 at, Outer, jjzjfc, 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-lay er r 

 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 th& 

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

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



