THE FCETAt, MEMBRANES OF REPTILES AND BIRDS. 217 



2. The Allantois. 



While the development of the amnion is still going on, there is 

 formed in Reptiles and Birds an embryonic organ of no less import- 

 ance, the allantois, or urinary sac. It has two different functions 

 to perform at the same time. In the first place it serves, as its 

 name implies, for the reception of the excretory products which are 

 furnished during embryonic life by the kidney and primitive kidney ; 

 and secondly, by virtue of the abundance of blood-vessels and the 



via 



Fig. 128.- -Diagrammatic longitudinal section through, the posterior end of an embryo Chick at 

 the time of the formation of the allantois, after BALFOUR. 



The section shows that the neural tube, Sp.c, is continuous at its posterior end with the hind 

 gut, p.a.g, by means of the neurenteric canal, n.e. The latter passes through the remains of 

 the primitive streak, pr, which is folded over toward the ventral side, ep, Outer germ-layer ; 

 eh, chorda ; hy, entoderm (hypoblast) ; al, allantois ; me, middle germ-layer ; ore, the point 

 where the anus will arise ; am, amnion ; so, somatopleure ; gp, splanchnopleure. 



superficial position that it acquires, it is the most important organ of 

 respiration. 



The allantois takes its origin from the posterior portion of the 

 hind gut, which is afterwards designated as the cloaca, and in the 

 Chick the first traces of it can be recognised even at the end of 

 the second day, at a time when the walls of the hind gut are still 

 in the process of formation. It appears in this instance as a small 

 ceecal evagination (al) on the anterior wall of the splanchnopleure 

 (Ay) (fig. 128; Plate I., fig. 3 al). 



The evagination is lined by the entoderm, and is covered exter- 

 nally by a growth of the splanchnic mesoderm. It enlarges rapidly 

 into a vesicle, which grows out into the body-cavity (Plate I., fig. 4 al). 

 At the same time the blind end enlarges, whereas the proximal part, 

 where it passes over into the hind gut, becomes narrow and elongated 

 into a hollow stalk, the urinary duct or urachus. 



