250 ESSENTIALS OF ZOOLOGY 



the endoderm as the notochord to a further point in advance. 

 The primitive streak is accompanied by the formation of a 

 primitive groove, and it is continued as a canal at the anterior 

 end of the streak into the notochord. It opens also, sooner 

 or later, into the enteron, and ultimately forms the neurenteric 

 canal. Mesoderm is liberated on each side from the endoderm 

 continuous with the notochord and also with that formed 

 posteriorly by the primitive groove. The neural plate is 

 distinct in front of the streak. The conditions, in short, are 

 directly comparable with those of the bird, still more with 

 those of the reptile. 



As has been seen, the blastopore is late in appearing in 

 the case of the chick, and thus evidences the persistence of 

 a process no longer necessary in connexion with gastrulation, 

 but essential in association with postgastrular growth. It is 

 also late in appearance in mammals. 



The preblastoporal area of the embryo constantly increases 

 in length, and, as it grows, the structures undergo changes 

 already familiar from the study of the chick. 



The neural plate becomes denned by neural ridges which 

 limit the brain in front and extend backwards as folds of the 

 ectoderm. These folds continue to rise and they meet and fuse, 

 leaving a neuropore in front, and extend backwards, fusing as 

 growth proceeds. A neural sinus is left for a time posteriorly, 

 enclosing the front end of the streak. But this is duly roofed 

 over, and the front end of the blastopore is converted into the 

 neurenteric canal. 



Head folds are also formed as in the bird, and their formation 

 is accompanied by the head being raised from the area pellucida 

 and the fore-gut being established. The folds are continued 

 backwards, and are met by a tail fold. The enteron is thus 

 converted into fore-, mid-, and hind-gut, the mid-gut still com- 

 municating widely with the yolk-sac vesicle. The expansion 

 of the head is accompanied by a mesencephalic flexure, followed 

 by a flexure of the head as a whole. This forms a pit into the 

 area pellucida in front of the embryo, and the two layers 

 there is no mesoderm at this stage in this region are carried 

 downwards. In the case of the mammals which form an 

 amnion after the manner of birds and reptiles, the outer wall 



