i66 



PRINCIPLES OF EMBRYOLOGY 



which Ues shghtly more laterally also accumulates towards the midline, 

 forming two strips of thickened mesoderm, which soon become seg- 

 mented transversely, to form two rows of more or less cubical blocks, the 

 somites. 



All these changes begin at or near the anterior end, and progress steadily 

 posteriorly; though it should be noticed that the neural groove, being 

 originally wider at its anterior end, does not succeed in closing over so 

 early in its widest parts as it does somewhat further back. At the most 

 posterior end of the embryonic axis, the remains of the blastopore persist 



Figure 9.10 



Sagittal sections at three stages {a, b, c,) of gastrulation in a urodele, to show 

 the expansion of the archenteron and the obliteration of the blastocoel ; the 

 extent of the lateral mesoderm is indicated by shading. Below, J is a dorsal 

 view on to the blastopore (presumptive notochord closely dotted, presump- 

 tive neural system dashed); e is a yolk plug stage;/, the first appearance of 

 the neural plate ;^, the neural fold stage. Note that in /considerable meso- 

 derm, and even some notochord, is still on the surface, while even in^ there 

 is a little mesoderm between the neural folds near the remnant of the blasto- 

 pore. (From Pasteels 1940, after Vogt and Nakamura.) 



as a narrow slit; and here, as was hinted above, conditions are still much 

 the same as they were in the more widely open blastopore of the yolk-plug 

 stage, and gastrulation movements still proceed, although on a smaller 

 scale. The notochord, at the dorsal apex of the blastopore, is the first to 

 follow the endoderm completely below the surface, but the invagination 

 of mesoderm round the lateral lips persists for some time longer. Vital 

 staining demonstrates, in fact, that by the first appearance of the neural 



