74 TEXT-BOOK OF EMBRYOLOGY. 



separation of the mesoderm extends dorsally to the chorda and ventrally, as 

 indicated by arrows in Fig. 63, splitting off the superficial cells of the yolk 

 entoderm until the mesoderm becomes completely separated from the yolk cells. 

 On each side of the notochord the mesoderm shows a shallow longitudinal groove 

 (Fig. 64) which has been interpreted by some as the homologue of the meso- 

 dermic evagination of Amphioxus. This groove does not persist, however, and 

 has nothing to do with the formation of the body cavity. The latter in the frog 

 results not from evagination but from a splitting of the originally solid mesoder- 

 mic plates. It is to be noted, however, that while the ccelom does not originate 

 as an evagination from, and is never connected with, the primitive intestine, 

 the mesoderm itself consists of cells which have split off from the wall of the 



Chorda anlage 



f Ectoderm 



Yolk entoderm 



Remnant of 

 segmentation cavity 



FIG. 63. Transverse section of embryo of frog (Rana fusca). Bonnet. The section is taken in front 



of (anterior to) the blastopore. 



primitive intestine (entoderm), and that it is within this group of cells that the 

 ccelom finally appears. Of the yolk cells, only the outermost (most peripheral) 

 have to do with the formation of intestinal epithelium, the remainder being 

 ultimately used up for the nutrition of the embryo (Fig. 65). 



The formation of the neural groove and neural tube from the ectoderm and 

 the separation of the chorda anlage from the rest of the entoderm are much the 

 same as in Triton. 



Mesoderm Formation in Reptiles and Birds. The actual origin of 

 the mesoderm in these forms is very difficult to determine owing to the pecu- 

 liarities of gastrulation which in turn are due to the greatly increased amount 

 of yolk. In the lower forms it has been seen that the mesoderm is primarily a 

 derivative of the entoderm (Amphioxus, Fig. 56), or of protentoderm and yolk 



