DEVELOPMENT OF THE FCETUS. 



39 



becomes necessary that the fostus F>g- 12/ 



should have some other means of ^• 



support, and the following are sup- 

 plied. From the inferior or caudal 

 extremity of the foetus there arises 

 at the point where the urinary blad- 

 der is afterwards seated, a delicate 

 membranous sac, which is formed 

 from the mucous layer, and has dis- 

 tributed upon its surface, a number 

 of minute blood-vessels, from the 

 vascular layer ; this vesicle gradu- 

 ally extends itself between the am- 

 nion and the chorion, until it comes 

 in contact with that part of the uterine surface, where the villi of the 

 chorion are most abundant ; here the vessels which it carries, attach 

 themselves in order to form the placenta, and the allantois having 

 fulfilled its function shrivels up, although traces of it may be found 

 in the cord. In its development the allantois passes out of the ante- 

 rior part of the abdominal cavity, in the region of the umbilicus, 

 which at this time is open. As the cavity is closing, by the gradual de- 

 velopment of its walls towards the median line, it is separated into 

 two portions which communicate; that part which is within the body, 

 forms the urinary bladder, with its urachus or tube of communication. 

 (Fig. 13.) ^ 



The Amnion, — The outer or serous layer of the germmal mem- 

 brane at the point where the embryo is developing itself, rises up on 

 each side of it in two folds, 

 forming the inner membrane of 

 the ovum, which is called the 

 amnion. These two folds gra- 

 dually approach each other till 

 actual contact takes place, in 

 the space between the general 

 envelope and the embryo, so as 

 to form an additional investment 

 to the latter. As each fold con- 

 tains two layers of membrane, 

 a double envelope is thus formed, 

 of which the outer layer (fig. 

 12, d, <?,) afterwards adheres to 

 the inner surface of the chorion; 



* a, chorion ; 6, umbilical vesicle, surrounded by the serous and vascular laminse; c, em- 

 bryo ; rf, e, and /, external and internal folds of the serous layer, forming the amnion ; g, in- 

 cipient allantois. 



t Diasram representing a Human Ovum in second month ; a, 1, smooth portion of chorion; 

 a, 2, villous portion of cJiorion ; k, k, elongated villi, beginning to collect into placenta; b. 

 yolk-sac or umbilical vesicle ; c, embryo ; /, amnion (inner layer) ; g, allantois ; h, outer 

 layer of amnion, coalescing with chorion. 



Fig. 13.t 



