94 



TEXT-BOOK OF EMBRYOLOGY. 



gather while the ectoderm is a separate layer (Fig. 65, Si and 52). This 

 disposition of the layers is characteristic of the primitive axis in the bird. 



Ectoderm 



Si 



Mesoderm 



Yolk entoderm 



Pr.int.co. 

 P.gr. Ectoderm 



Yolk entoderm Pr.st. 



FIG. 65. Transverse sections of embryonic disk of dog. Bonnet. 



Sections of disk shown in Fig. 64. Letters and numbers at right (Si-S 4 ) indicate plane of sections 

 in Fig. 64. P.gr., Primitive groove; Pr.int.co., primitive intestinal cord; Pr.st., all three 

 germ layers fused in primitive streak. 



Primitive streak Entoderm Mesoderm Ectoderm 



FIG. 66. Transverse sections of embryonic disks of rabbit, (a) Kolliker, (b) Rabl. 



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. 



(Compare Fig. 48.) The conditions as portrayed in the rabbit (Fig. 66, 



