128 



EMBRYOLOGY 



Fig. 71. Two diagrams showing 

 the progressive migration of the 

 mesoderm and endoderm during 

 neurulation. Above: Neurula with 

 open neural plate and early neural 

 ridges, C, C t . Below the neural plate 

 are the notochord, N, and the lateral 

 mesoderm, M, M v M and M 1 are 

 migrating ventrally. The endoderm, 

 E, E x extends dorsally. Right: A 

 later neurula in which the neural 

 ridges approximate each other. The 

 notochord becomes segregated from 

 the mesoderm. E and £ x have met 

 and fused, thus completing the'roof 

 of the gut G. A is the archenteron. 



C C| 



the presumptive notochord, N, and the presumptive lateral mesoderm, M, M x , 

 to the presumptive endoderm E, E lt E f , as shown on the external surface of 

 an early gastrula (Fig. 69). The cells comprising these regions must take an 

 internal position during gastrulation. (Figure 70) is a cross section showing 

 the regions after gastrulation has taken place. An archenteron has formed 

 with N as roof and E f as floor. The left and right walls of the archenteron 



