i>i:\i Of MM: i\v MIIUH.I: <.r.i;\i i. \VKU8. 131 



A Mmilar vie\v is furni>ht d l>y a cross section through the cephalic 

 process of the germ of the Chick (fig. ( J.'{). I'ndrr the outer germ- 

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

 groove, only the fundament of the chorda (ch) ; at t he point indicated 

 by a Mar it is continued laterally into tiie Mnall-celled middle gi 

 layer, and into the ento.lerm, which N eompox-d of a single layer of 

 very much tlat tei.ed cells. 



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

 iing 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 HEAPE is a single layer of cylindrical cell> ; it ha- 

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

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

 mass of small cells, which is resolved into two layer* 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 IL After HK.U-K. 

 The section passes through the chordal groove (<-h) somewhat farther forward than the section 



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



blastopore. 

 ak, Outer, ink, middle, it, inner germ-l.tyer ; ch, fundament <>f 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-1 

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



In a still more convincing manner VAN BENEDKN 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 Rabbit, taken from 

 his work, is especially convincing. It >ho\vs the fundament of th-- 

 chorda (ch) as a single layer of cylindrical celN, flanked on the right 

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

 layer consists of a parietal (tnk ] ) and a visceral (mk-) 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 



