DEVELOPMENT OF THE EMBRYO. 



717 



and distinctness, and become sufficiently powerful to propel the blood through 

 the vessels of the Embryo and those of the Vascular Area. The first ap- 

 pearance of the Heart in the Chick is at about the 27th hour ; the time of its 

 formation in Mammalia has not been distinctly ascertained. In its earliest 

 form, it has the same simple character, which is presented by the central im- 

 pelling cavity of the lower Invertebrata ; being a mere prolonged canal, which 

 at its posterior extremity receives the veins, and at its anterior sends forth the 

 arteries. After a short time, however, it becomes bent upon itself (Plate II., 

 Fig. 27, d] ; and it is soon subdivided into three cavities, which exist in all 

 Vertebrata, a simple auricle or receiving cavity, a simple ventricle or pro- 

 pelling cavity, and a bulbus arteriosus at the origin of the aorta. The circu- 

 lation is at first carried on exactly upon the plan, which is permanently 

 exhibited by Fishes. The Aorta subdivides into four or five arches on either 

 side of the neck ; and these are separated by slits or fissures, much resem- 

 bling those which form the entrances to the gill-cavities of Cartilaginous 

 Fishes. These arches reunite to form the descending aorta, which transmits 

 branches to all parts of the body. Such is the first phase or aspect of the 

 Circulating 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 num- 

 ber 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. 



941. With the evolution of a Circulating apparatus, adapted to absorb 

 nourishment from the store prepared for the use of the Embryo, and to con- 

 vey it to its different tissues, it becomes necessary that a respiratory apparatus 



Fig. 285. 



Fig. 286. 



.-* 



The Amnion in process of formation, by the 

 arching over of the serous lamina; a, the 

 chorion ; 6, the yolk-bag, surrounded by se- 

 rous and vascular lamina ; e, the embryo ; 

 d, , and/, external and internal folds of the 

 serous layer, forming the amnion ; g, incipi- 

 ent allantois. 



Diagram representing a Human Ovum in' 

 second month; a. I, smooth portion of cho- 

 rion ; a. 2, villous porlion'of chorion ; k, k, 

 elongated villi, beginning to collect into Pla 

 centa; 6, yolk-sac or umbilical vesicle; c, em 

 bryo ;/, amnion (inner layer); g-, allantois; 

 h, outer layer of amnion, coalescing with 

 chorion. 



