CLEAVAGE AND THE GERM LAYERS 



incorporate the blastopore into its floor. This temporary communica- 

 tion between the neural tube and the primitive enteric cavity is the neuren- 

 teric canal (cf. Fig. 21 C) ; it is found in all the vertebrate groups (cf. Fig. 78). 

 A transverse section through the invaginated pouch, at the time of 

 rupture of its floor, and the underlying entoderm will make clear the 

 relatively slight lateral extent of these changes (Fig. 22). 



From about the blastopore, and from the walls of the pouch, mesoder- 

 mal plates arise and extend like wings between the ectoderm and entoderm 

 (Fig. 22). As in amphibia, they later separate into outer (somatic) and 

 inner (splanchnic) layers enclosing the coelom (cf. Fig. 29 B). The rela- 

 tion between notochordal plate, 

 mesoderm, and entoderm shown in 

 Fig. 22 resembles strikingly the con- 

 ditions in Amphioxus (Fig. 20 A). 



Birds. Due to the modified 

 gastrulation in reptiles, birds, and 

 mammals through the influence of 

 yolk, a structure known as the 

 primitive streak becomes important. 

 An account of its formation and 

 significance based on conditions 

 found in the bird may be introduced 

 conveniently at this place. 



Shortly after the formation of 

 entoderm there appears in the 

 median line at the more caudal por- 

 tion of the blastoderm an elongated opaque band (Fig. 23). Along 

 this primitive streak, which is at first merely a linear ectodermal thickening, 

 there forms a shallow primitive groove, bounded laterally by primitive 

 folds. At its forward end the groove enters a depression, the primitive pit. 

 In front of this pit the streak ends in a knob, the primitive knot, or node 

 (of Hensen). 



The primitive streak becomes highly significant when interpreted in 

 the light of the theory of concrescence, a theory of general application in 

 vertebrate development. It will be remembered that the entoderm of 

 birds arises by a rolling under of the outer layer along the caudal margin of 

 the blastoderm. As the blastoderm expands, it is believed that a middle 

 point on this margin remains fixed while the edges of the margin on each 

 side are carried caudad and brought together. Thus, a crescentic margin 

 is transformed into a longitudinal slit, as in Fig. 24. Since this marginal 

 lip originally bounded the blastopore (p. 29), the longitudinal slit must 

 also be an elongated blastopore whose direction has merely been changed. 



Area opaca 



Primitive knot 

 Primitive pit 



Primitive fold 

 Primitive groove 



Area pellncida 

 Blood island 



FIG. 23. Blastoderm of a chick embryo 

 at the stage of the primitive streak and 

 groove (16 hours). X 20. 



