992 OF GENERATION- 



as the temporary stomach of the embryo remaining as the permanent stomach 

 in the Radiated tribes. 1 



1000. The formation of the Heart, which is the first of the permanent organs 

 of the Embryo that comes into functional activity, takes place in the substance 

 of the vascular layer, beneath the upper part of the spinal column. Its first 

 rudiment consists of an aggregation of cells, of which the inner part break down 

 to form its cavity, whilst the outer remain to constitute its walls. For a long 

 time after it has distinctly commenced pulsating, and is obviously exerting a 

 contractile force, its walls obviously retain the cellular character, and only be- 

 come muscular by a progressive histological transformation ( 310). The first 

 appearance 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 impelling 

 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. 13, d)j 

 and it is soon subdivided into three cavities, which exist in all Vertebrata, viz., 

 a simple auricle or receiving cavity, a simple ventricle or propelling cavity, and 

 a bulbus arteriosus at the origin of the aorta. The circulation 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 (Figs. 266, 267, e, 

 ef, e"), which are separated by fissures much resembling those forming the en- 

 trances to the gill-cavities of Cartilaginous Fishes ; and 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 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 pro- 

 duction of new vessels ; these changes will be presently described. It is in- 

 correct, 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 Foatus towards the 

 end of the first month ; during the second, they usually close up and disappear. 



1001. With the evolution of a Circulating apparatus, adapted to absorb 

 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 



1 Previously to the ninth day of incubation (in the Fowl's egg), a series of folds are 

 formed by the lining membrane of the yelk-bag, which project into its cavity ; these become 

 gradually deeper and more crowded, as the bag diminishes in size by the absorption of 

 its contents. The vitelline vessels that ramify upon the yelk-bag send into these folds 

 (or valvulae conniventes) a series of inosculating loops, which immensely increase the ex- 

 tent of this absorbing apparatus. But these minute vessels are not in immediate contact 

 with the yelk ; for there intervenes between them (as was first noticed by Mr. Dalrymple) 

 a layer of nucleated cells, which is easily washed away. (See Dr. Baly's Translation of 

 " M tiller's Physiology," pp. 1557-1559.) It was from the color of these, communicated 

 to the vessels beneath, that Haller termed the latter vasa lutea ; when the layer is removed, 

 the vessels present their usual color. There seems good reason to believe that these 

 cells, like those of the Intestinal Villi in the adult ($ 461), are the real agents in the pro- 

 cess of absorbing and assimilating the nutritive matter of the yelk ; and that they deliver 

 this up to the vessels, by themselves undergoing rupture or dissolution, being replaced by 

 new layers. 



