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TEXT-BOOK OF EMBRYOLOGY. 



layers fused beneath the primitive groove (Fig. 75, S 3 and S 4 ; compare with 

 Fig. 66). The origin of the mesoderm is probably, as in the chick, to be at- 

 tributed to the invaginated cells (protentoderm) along the line of the primitive 

 groove. The mesodermal cells first appear as a small mass beneath the primi- 

 tive groove (Fig. 76, a) ; they then spread out laterally between the ectoderm and 

 (yolk) entoderm (Fig. 76, b). Beneath the point of origin, that is, along the 



Primitive streak Entoderm Mesoderm Ectoderm 



FIG. 76. Transverse sections of embryonic disks of rabbit, (a) Kolliker, (b) Rail, 

 a, section through primitive streak of embryo of 6 days and 18 hours; b, section through Hensen's 

 node of embryo of 7 days and 3 hours. 



line of the primitive groove, they finally fuse with the (yolk) entoderm (Figs. 

 75, S 3 and S 4 ; compare Figs. 76, a and b, and Figs. 75, S 3 and S 4 with Figs. 69, 

 70 and 66). 



In the region of the head process, as in the chick, sections show at first the 

 entoderm and mesoderm fused in the medial line, and the ectoderm as a sepa- 

 rate layer (Fig. 77 and Fig. 75, S 2 ). The entoderm with which the mesoderm is 



Ectoderm 



Mesoderm Notochord 

 I I 



Entoderm 



FIG. 77. Transverse section of embryonic disk of rabbit, van Beneden. 



fused represents the invaginated cells, that is, the protentoderm ("primitive 

 intestinal cord"); and, as in the chick, it seems reasonable to assume that the 

 mesoderm is derived from the "primitive intestinal cord" (protentoderm) and 

 grows out laterally between the ectoderm and entoderm (compare Fig. 75, S 2 

 with Fig. 67). 



A little later, in the region of the head process, the mesoderm on each side is 



