FORMATION OF VESSELS AND DIGESTIVE CAVITY. 



459 



814. The formation of the Heart takes place in a thickened portion 

 of the Vascular layer ; by the liquefaction of the interior of a mass of 

 cells, of which the exterior constitute the first walls of the cavity. 

 These gradually acquire firmness and consistency, and are endowed 

 with a contractile power that enables them to execute regular pulsa- 

 tions. In this early condition, the heart is known as the punctum 

 saliens (d, Fig. 142). 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. 



815. Concurrently with the formation of the Vascular system, the 

 production of the permanent Digestive cavity commences. This origi- 

 nates in the separation of a small portion of the yolk-bag, lying imme- 

 diately beneath the embryo, from the general cavity, in the following 

 manner. At about the 25th hour of incubation, in the Fowl's egg, the 

 parts of the germinal membrane which lie beyond the extremities, and 



Fig. 143. 



Fig. 144. 



Diagram of Mammalian Ovum at later stage ; 

 the digestive cavity beginning to be separated 

 from the yolk-sac, and the amnion beginning to 

 be formed: a, chorion; 6, yolk-sac; c, embryo; d 

 and e, folds of the serous layer rising up to form 

 the Amnion. 



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/, ex- 

 ternal and internal folds of the serous layer, 

 forming the amnion; g, incipient allantois. 



which spread out from the sides of the embryo, are doubled-in, so as to 

 make a depression upon the yolk ; and their folded edges gradually 

 approach one another under the abdominal surface of the embryo, so as 

 at last to meet and enclose a cavity, which is at/first simple in its form, 

 but which is subsequently rendered more complex by the prolongation 

 and involution of its walls in various parts, so as to form the stomach 

 and intestinal tube (Figs. 143, 144, 145). This digestive cavity com- 

 municates for some time with the yolk-bag (from which it has been thus 

 pinched-off, as it were), by a wide opening, that is left by the imperfect 

 meeting of the folds of the germinal membrane that constitute its walls. 

 In the Mammalia, this orifice is gradually narrowed, and is at last com- 

 pletely closed ; and the yolk-bag, thus separated, is afterwards thrown 

 off. It may be detected, however, upon the umbilical cord, up to a late 

 period of pregnancy, and is known as the Umbilical vesicle (Fig. 146, t). 

 In Birds, and other oviparous animals, the whole of the yolk-bag is 



