GERM LAYERS. 



87 



found to be separated from the parent tissue ("primitive intestinal cord"), and 

 the latter now represents the anlage of the notochord (compare Fig. 72 with 

 Fig. 78). 



On the ground that the primitive groove is the blastopore, the mesoderm 

 arising in that region is the peristomal mesoderm; that arising from the 

 "primitive intestinal cord" in front of the primitive groove is the gastral meso- 



Mesoderm Ectoderm Neural groove 

 i 



408^ 



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Yolk entoderm 



Chordal plate 



FIG. 78. Transverse section of embryonic disk of dog. Bonnet. 

 Section taken near anterior end of head process. 



derm. The peristomal and gastral portion together constitute a continuous 

 plate of cells interposed between the ectoderm and entoderm, which has been 

 derived from the invaginated cells of the protentoderm. 



In a few Mammals (sheep, roe, shrew), mesoderm has been seen to arise 

 some distance from the primitive streak and head process (Fig. 79). This has 

 been called the peripheral mesoderm, but it soon unites with the peristomal and 

 gastral. 



Embryonic disk 



Area of 



invagination 



Nuclei of 

 yolk entoderm 



Ectoderm 



FIG. 79. Surface view of embryonic disk of sheep. Bonnet. 

 Disk is at that stage of development when gastrulation begins (in region marked area of invagination). 



Primarily, the mesoderm is a solid plate of cells with no indication of a body 

 cavity (ccelom). A little later the mesoderm splits into two layers, the parietal 

 and the visceral, between which lies the ccelom (Fig. 81). The splitting does 

 not effect, however, the mesoderm which lies adjacent to the neural groove on 

 both sides of the medial line, for this portion becomes differentiated into two 

 series of bilaterally symmetrical segments the primitive segments (Figs. 80 and 



