206 



REPTILIA. 



Thus it happens that the central digestive canal, which at first has 

 the form of a shallow groove, becomes converted into a tube which 

 remains for some time connected with the yolk at the umbilicus by 

 a narrow duct. 



The appearance of a membrane enclosing the embryo and known 

 as the amnion (fig. 635) is characteristic. The amnion arises in the 

 following way. The outer layer (somatopleure) of the blastoderm is 

 raised at the anterior and posterior end of the embryo, and forms 

 two folds covering the head and tail end. These folds soon extend 



over the lateral por- 

 tions, and fuse over 

 the body of the em- 

 bryo, so as to form 

 a closed sac filled 

 with fluid. Another 

 organ which is cha- 

 racteristic of the 

 higher vertebrates 

 is the allantois. This 

 arises at the pos- 

 terior end of the 

 body as a vesicular 

 evagination of the 

 ventral wall of the 

 alimentary canal, 



Fio. 635. Two stages in the development of the chick (after v. and gl'OWS Out to 



. , 



ot COnsidera- 



size. The walls 



, . , . , 



tn lS sac, wnicn 



Baer) to show the development of the amnion and the 



allantois.-a, The two folds of the amnion are still widely 



separate from one another ; only the first rudiments of the 



allantois are visible. b, Later stage with closed amnion. E, 



embryo ; D, viteUine membrane ; Am, amnion Sh, Serous 



membrane ; Dh, alimentary cavity ; Dg, umbilical passage ; j s filled with fluid 



r, Yolk ; C, heart Al, allantois. _ .. 



are, unlike those of 



the amnion which is entirely without vessels, extraordinarily vascular 

 and represent an embryonic respiratory organ, which in the long 

 duration and complicated developmental processes of embryonic life 

 is of great importance. The appearance of the allantois is correlated, 

 not only with the disappearance of branchial respiration, but also 

 with the complete absence of a metamorphosis; the young animal 

 being completely organised when it leaves the egg. 



Some Snakes and Lizards extend far north, while the Crocodilia 

 are confined to the torrid zone, and only isolated examples of the 

 Chelonia belong to the torrid zone. 



