GERM LAYERS. 77 



and appears in connection with the " primitive intestinal cord/' it is the gastral 

 mesoderm, the homologue of the gastral mesoderm described in lower forms 

 (Fig. 63). Here also, as in the case of the peristomal mesoderm, the mesoderm 

 is primarily a solid plate of cells. Furthermore, immediately in front of the 

 primitive groove the gastral mesoderm is continuous with the peristomal. 



At a still later stage the gastral mesoderm is found to be separated from the 

 entoderm, so that the "primitive intestinal cord" (now the notochord) separates 

 the mesoderm of the two sides in the medial line (Fig. 68). 



Neural plate Xotochord 



' Ectoderm 

 ' Mesoderm 



:-" ' Entoderm 

 "" " Archenteron 



FIG. 68. Transverse section of blastoderm of chick (40 hours' incubation). Hertwig. 

 Section taken short distance anterior to Hensen's node. 



Comparing the conditions in sections through the head process in the chick 

 with sections through the body region of the frog (Figs. 63 and 64), a fairly 

 clear homology may be drawn. 



While in the stages just described in the chick the mesoderm is present and 

 interposed between the ectoderm and entoderm, the crucial point is its actual 

 origin. In the lower forms it originated from the entoderm, that is, from the 

 cells which have been invaginated at the blastopore. In the chick the blasto- 

 pore ; which is crescent-shaped, is transformed into a longitudinal structure 



Mesoderm Primitive groove 



_ FIG. 69. Transverse section of blastoderm of chick (10 hours' incubation). Hertwig. 

 Section taken through primitive groove and streak. 



the primitive groove but still the blastopore. As the crescentic blastopore 

 becomes longitudinal, the two horns come together and fuse (see p. 61), and 

 the line of fusion still represents the area of imagination, where some of the 

 surface cells have grown under the remaining surface cells to form the entoderm 

 (protentoderm) . And it is along this area of invagination that the mesoderm 

 first appears. In very early stages there is an especially active cell proliferation 

 in the thickened layer of cells which represents the primitive streak. This 

 activity gives rise to a mass of cells which lie immediately beneath the primitive 



