FORMATION OF HEART AND FIRST BLOODVESSELS. 



937 



nourishment from the .store prepared for the use of the Embryo, and to convey 

 it to its different tissues, it becomes necessary that a Respiratory apparatus 

 should also be provided, for depurating the blood from the carbonic acid 

 with which it becomes charged during the course of its circulation. The 

 temporary Respiratory apparatus now to be described, bears a strong resem- 

 blance in its own character, and especially in its vascular connections, to the 

 gills of the Mollusca ; which are prolongations of the external surface (usu- 

 ally near the termination of the intestinal canal), and almost invariably re- 

 ceive their vessels from that part of the system. This apparatus, which is 

 termed the Allantois, sprouts forth from the splanchnopleure close to its 

 junction with the somatopleure, at first as a little mass of cells, which soon 

 exhibits a cavity (probably originating in the liquefaction of the cells of the 

 internal part), so that a vesicle is formed (Figs. 342, 343, g), which looks like 



FIG. 342. 



FIG. 343. 



^% 



^v> 



FIG. 342. Diagram of an early Human Ovum, showing the Amnion in process of formation and the 

 Allantois beginning to appear : a, chorion ; 6, vilellinf mass surrounded by the blastodermie vesicle; 

 c, embryo ; d, e, and/, external and internal folds of the serous layer, forming the amnion ; g, incipi- 

 ent allantois. 



FIG. 343. Diagram of a Human Ovum in second month, showing the completion of the sac of the 

 Amnion, and a fu:ther development of the Allantois : a 1, smooth portion of chorion ; a 2, villoua 

 portion of chorion ; k, k, elongated villi, beginning to collect into Placenta; 6, vitelline or umbilical 

 vesicle ; c, embryo ; /, amnion (inner layer); g, allantois ; h, outer layer of amnion, coalescing with 

 chorion. 



a diverticulum from the lower part of the digestive cavity. The outer cells 

 are composed of mesoblast, the internal of hypoblast. This vesicle, in 

 Birds, has been shown by Vulpian 1 to be possessed of a distinct contractile 

 power, and soon becomes so large as to extend itself around the whole yolk- 

 sac, intervening between it and the membrane of the shell, and coming 

 through the latter into relation with the external air; but in the embryo of 

 Mammalia, the allautois, being early superseded by another provision for 

 the aeration of the blood, seldom attains any considerable dimensions. Its 

 chief office here is to convey the vessels of the embryo to the Choriou ; and 

 its extent bears a pretty close correspondence with the extent of surface 

 through which the Chorion comes into vascular connection with the decidua. 

 Thus, in the Carnivora, whose placenta extends like a baud around the 

 whole ovum, the allantois also lines nearly the whole inner surface of the 

 chorion ; on the other hand, in Man and the Quadrumaua, whose placenta is 

 restricted to one spot, the allantois is small, and conveys the ftetal vessels to 

 one portion only of the chorion. When the vessels have reached the choriou, 



1 Journ. dc la Physiologie, torn, i, p. 619, et scq. 



60 



