DEVELOPMENT OF THE EMBRYO. 



993 



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

 Respiratory apparatus now to be described bears a strong resemblance in its 

 own character, and especially in its vascular connections, to the gills of the 

 Mollusca, which are prolongations of the external surface (usually near the 

 termination of the intestinal canal), and which almost invariably receive their 

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

 Allantois, sprouts forth from the caudal extremity of the embryo at first as a 

 little mass of cells, which soon exhibits a cavity (probably formed by the lique- 

 faction of the cells of the internal part), so that a vesicle is formed (Figs. 264, 

 265, </), which looks like a diverticulum from the lower part of the digestive 



Fig. 265. 



Diagram of an early Human Ovum, with the 

 Amnion in process of formation : a, the chorion ; 

 b, the vitelline mass, surrounded by the blasto- 

 dermic vehicle ; c, the embryo ; d, e, and /, ex- 

 ternal and internal folds of the serous layer, 

 forming the amnion ; g, incipient allantois. 



Diagram representing a Human Ovum in second 

 month: a 1, smooth portion of chorion; a 2, 

 villous 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. 



cavity. This vesicle, in Birds, soon becomes so large as to extend itself around 

 the whole yelk-sac, intervening between it and the membrane of the shell, and 

 coming through the latter into relation with the external air ; but in the em- 

 bryos of Mammalia, being early superseded by another provision for the aeration 

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

 chief office in them is to convey the vessels of the embryo to the chorion ; 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 band around the whole ovum, the 

 allantois also lines nearly the whole inner surface of the chorion ; on the other 

 hand, in Man and the Quadrumana, whose placenta is restricted to one spot, 

 the allantois is small, and conveys the foetal vessels to one portion only of the 

 chorion. When these vessels have reached the chorion, they ramify in its 

 substance, and send filaments into its villi ; and in proportion as these villi form 

 that connection with the uterine structure which has been already described 

 ( 977-982), do the vessels increase in size. They then pass directly from 

 the foetus to the chorion ; and the allantois, being no longer of any use, shrivels 

 up ; and remains as a minute vesicle, only to be detected by careful examina- 

 tion. The same thing happens in regard to the umbilical vesicle, from which 

 the entire contents have been by this time withdrawn ; and from henceforth the 

 foetus is completely dependent for the materials of its growth upon the supply it 

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