ORIGIN OF THE MESODERM, NOTOCHORD AND NEURAL TUBE 



31 



communication with the space (archenteron) beneath the entoderm (Fig. 21 C). 

 The cells of the roof persist as the notochordal plate which later gives rise to the 

 notochord. The neural folds arise before the mouth of the pouch (blastopore) 

 closes up, and, fusing to form the neural tube, incorporate the blastopore in its 

 floor. This temporary communication between the neural tube and the primitive 

 enteric cavity is the neurenteric canal (Fig. 2 1 C) ; it is found in al the vertebrate 

 groups (cf . Fig. 78) . A transverse section through the invaginated pouch, at the 

 time of rupture of its floor, and the underljdng entoderm will make clear the rela- 

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



From about the blastopore, and from the walls of the pouch, mesodermal 

 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 relation between notochordal 

 plate, mesoderm, and entoderm shown in Fig. 22 resembles strikingly the condi- 

 tions in Amphioxus (Fig. 20 ^). 



Ectoderm 



Mesoderm 



Entoderm 



|(.fjps?>r 



Notochordal 

 plate 



Fig. 22. — Transverse section of a snake's blastoderm at a level corresponding to the middle of Fig. 



21 C (adapted after Hertwig). 



Birds. — Due to the modified gastrulation in reptiles, birds, and mammals \ 

 through the influence of yolk, a structure known as the primitive streak becomes l 

 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 formatio n of entoderm there appears in the median line at 

 the more caudal po rtion of the blastoderm an elongated opaque band (Fig. 23). 

 Along this prim,ith)e. streak there forms a shallow primitive groov e, bounded later- 

 ally hy prvrmtMi£-folds. Cran ially the groove ends in a depression, the primitive 

 pit. In front of this pit the streak ends in a knob, the primitive knot (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 bej remembered tha t_t.he entoderm nf ViirHg arigpg hy a 

 rolling under of the outer layer along the cauda] mar gin of the blastoderm . As 

 the blastoderm expands it is believed that a middle point on this margin remains 



