338 



ZOOLOGY 



SECT. 



(prot. v.), and below it a cord of endoderm cells, the rudiment of 

 the notochord ; the general history of these parts has already been 

 sketched in the section on the Craniata, and further details will 

 be given in the account of the development of Birds, which agrees 



FIG. 949. A D, early stages in the development of the Alligator. A, early stage with em- 

 bryonic shield, primitive streak and blastopore ; , considerably later stage in which the 

 medullary groove has become f ormed, together with the head-fold of the embryo and the head- 

 fold of the amnion ; C, somewhat later stage with well-developed medullary folds and 

 medullary groove ; D, later stage in which the medullary groove has become partly closed in 

 by the medullary folds and in which six pairs of protovertebrse have become developed. 

 amn. amnion ; a. op. area opaca ; a. pel. area pellucida ; blp. blastopore ; emb. s. embryonic 

 shield; /. 6?-. fore-brain; h. br. hind-brain; hd. /. head-fold*; m. br. mid-brain; maL /. 

 medullary folds ; prot. v. protovertebrse. (After S. F. Clarke.) 



with that of Reptiles in all essential respects. Under the head 

 of Birds also will be found an account of the formation of the 

 characteristic foetal membranes, the amnion and the allantois, 

 which applies in all essential respects to the Reptilia as well. 



