620 DEVELOPMENT. [CHAP.XLIII. 



from this time it becomes smaller, and gradually disappears. 

 Mayer has, however, detected both the vesicle and its thread-like 

 pedicle at the full period of gestation. 



Development of the Allantois. At a very early period of develop- 

 ment of the mammalian embryo, a collection of cells makes its ap- 

 pearance upon the anterior surface of its caudal extremity. This 

 gradually increases in size, becomes flask-shaped, and a cavity in 

 the interior of the mass becomes visible. The vesicle thus formed 

 rapidly enlarges. It contains fluid ; and upon its surface, ves- 

 sels, which ultimately become the umbilical vessels, are seen rami- 

 fying. As it grows, these vessels are carried with it towards the 

 inner surface of the chorion. The vessels of the umbilical vesicle 

 waste with this structure ; while those conducted to the placenta 

 by the allantois, ultimately become the two umbilical arteries and 

 the umbilical vein. In the human embryo, the chief office of the 

 allantois seems to be that of conducting the vessels towards that 

 portion of the chorion which is to become the future placenta ; and 

 as soon as the connection between the foetus and the placenta is 

 established, which in the human embryo takes place between the 

 third and fourth weeks, the allantois is no longer distinguishable. 

 Besides this office, however, the allantois receives the secretion 

 from the temporary kidneys, or corpora Wolffiana, previous to the 

 formation of the permanent structures. 



In many of the lower animals, however, the allantois is developed to a 

 much greater extent than it is in man. In birds and in several mammalian 

 orders, it forms a very large sac, which completely surrounds the embryo ; 

 and in the ruminants it contains many quarts of fluid, towards the termina- 

 tion of intra-uterine life. 



The allantois in the chick is readily distinguished before the close of the 

 third day, and appears to be connected with the terminal portion of the intes- 

 tine. Reichert has carefully investigated its development ; and has shown, 

 that it is not developed from the intestine or from the membrana intermedia, 

 but arises from two masses of cells, situated at the posterior extremity of the 

 "Wolffian bodies, which afterwards coalesce, forming a pear-shaped mass, in 

 which a cavity soon manifests itself. Passing from the Wolffian bodies to 

 the two small masses above referred to, are two lines or threads, which ulti- 

 mately become the excretory ducts of the former organs. 



At an early period, the allantois communicates with a common cavity, or 

 cloaca, into which the ureters, the excretory ducts of the Wolffian bodies, and 

 those of the organs of generation open. This is called the sinus urino-genitalis. 

 The allantois grows very rapidly, and ultimately entirely envelopes the embryo 

 with its amnion and yolk, and becomes applied to the inner surface of the 

 membrane of the egg-shell. It is highly vascular, and is, in fact, the respira- 

 tory organ of the chick as long as it remains within the shell. 



The arrangement of the capillaries has been investigated by our friei 



