Mil | Q AM) HIHDS. 



2. The AUantois. 



21 



\Vhilethe development of the amnion i> still going on, th. 

 formed in Reptiles and l.Unl> an eml.ryonie organ of no !>* imp -ct 

 Mice, tin- itlhuittn*. or urinary sac. It lias t \vo ditl'erent functions 

 to pnt..nn at the same time. In the first place it serves, as it- 

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

 furnished dunni; embryonic life by the kidney and primitive kidney; 

 and secondly, by virtue of tin- ah;n ' blo;>d-ve^sels and the 



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.t. The latter passes through the remains of 

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

 ch, chorda : hy, entoderm (hypoblast) ; al, allantois ; me, middle germ-layer; an, the point 

 u lii-re the anus will arise ; am, amnion ; 90, somatopleure ; sp, 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 

 (hick 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 

 caecal evagination (al) on the anterior wall of the splanclmopleure 

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



The rvaiiinaiinu is lined by the entoderm, and is covered e.v 

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

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

 At the same time the blind end enlarges, when-as the proximal part, 

 where it passes over into the hind irtit. 1>. comes narrow and elongated 

 into a hollow stalk, the urinary duct or urachus. 



