DEVELOPMENT OF THE EMBRYO. 



615 



Apparatus, which is common to all Vertebrata during the earliest period of 

 their development, and which may therefore be considered as its most general 

 form. It remains permanent in the class of Fishes ; and in them the vascular 

 system undergoes further development on the same type, a number of minute 

 tufts being sent forth from each of the arches, which enter the filaments of the 

 gills, and serve for the aeration of the blood. In higher Vertebrata, however, 

 the plan of the circulation is afterwards entirely changed, by the formation of 

 new cavities in the heart, and by the production of new vessels ; these changes 

 will be presently described. It is incorrect, therefore, to speak of the vascular 

 arches in their necks as branchial arches ; since no branchiae or gills are ever 

 developed from them. The clefts between them may be very distinctly seen 

 in the Human Foetus towards the end of the first month ; during the second, 

 they usually close up and disappear. 



764. With the evolution of a Circulating apparatus adapted to absorb nour- 

 ishment 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 unloading the blooo\ of the carbonic acid with which it 

 becomes charged during the course of its circulation. The temporary Respi- 

 ratory apparatus now to be described, bears a strong resemblance in its own 

 character, and especially in its vascular connections, with 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 is termed the Jlllantois. 

 It consists at first of a kind of diverticulum or prolongation of the lower part 

 of the Digestive' cavity, the formation of which has been already described. 

 This is at first seen as a single vesicle, of no great size (Fig. 185, g) ; and in 

 the Foetus of Mammalia, which is soon provided with other means of aerating 

 its blood, it seldom attains any considerable dimensions. In Birds, however, 

 it becomes so large as to extend itself around the whole Yolk-sac, intervening 

 between it and the membrane of the shell ; and through the latter it comes 

 into relation with the external air. The accompanying diagram will serve to 

 explain its origin and position in the Human ovum. The chief office of the 

 Allantois in Mammalia is to convey the vessels of- the embryo to the Chorion ; 



Fig. 184. 



The Amnion in process of formation, by the 

 arching over of the serous lamina; a, the cho- 

 rion; 6, the yolk-bag, surrounded by serous 

 and vascular laminae; c, the embryo; d, e and 

 f, external and internal folds of the serous 

 layer, forming the amnion: g, incipient allan- 

 tois. (After Wagner.) 



Diagram representing a Human Ovum in second 

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

 portion of chorion ; fc, &, elongated villi, beginning to 

 collect into placenta; b. yolk-sac or umbilical vesicle; 

 c, embryo; /, amnion (inner layer); g, allantois; A, 

 outer layer of amnion, coalescing with chorion. (After 

 Wagner.) 



