THE EMBRYONIC HEART. 971 



also this projects free, with the formation of the tail-fold (S) and the 

 hind-gut (d), to which the posterior intestinal portal leads. The em- 

 bryonal body thus communicates with the germinal vesicle by means 

 of a pedicle that is as first wide open. This pedicle is known as the 

 omphalomesenteric or vitellointestinal duct. The saccular vesicle 

 attached to it is designated in mammals the umbilical vesicle (VII, N), 

 while the analogous much larger sac in birds, which contains nourishment 

 from the yellow yolk, is known as the yolk-sac. Toward the end of the 

 third month of pregnancy the entodermal lining of the human umbilical 

 vesicle develops genuine liverlike glandular tissue. The omphalomes- 

 enteric duct becomes in its further course narrower and finally is oblit- 

 erated in the chick on the fifth day. Where the duct is inserted into the 

 abdominal wall there results the abdominal umbilicus ; where it is inserted 

 into the primitive gut there results the intestinal navel. 



On the ventral surface of the fore-gut and the hind-gut there are points where 

 the mesoderm is wanting, and where, therefore, the epiblast and the entoblast 

 come in contact. These are known as the pharyngeal and the cloacal membrane. 

 The openings for the formation of the oral and the anal orifices are later found 

 in these situations. 



Even before this process of constriction takes place the primitive 

 heart develops from that portion of the splanchnopleure that is in con- 

 tact below with the fore -gut, in the chick at the conclusion of the first day 

 as a rhythmically moving point (<m'/7y -^wnu^i^ of Aristotle, punctum 

 saliens); in mammals however, much later. The heart (Fig. 376, VI) 

 develops as a cellular, hollow, bladderlike bud of the splanchnopleure 

 (originally as a paired structure). Its cavity soon dilates and it grows 

 into the ccelom suspended from a mesentery -like duplicature (mesocar- 

 dium) : that part of the ccelom situated in the vicinity of the heart is now 

 designated the cardiac fossa (fovea cardiaca). The heart acquires a 

 longitudinal tubular form, with its aortic portion directed anteriorly 

 and its venous portion directed posteriorly. It then undergoes a mod- 

 erate S-shaped curvature (Fig. 384, i). From the middle of the second 

 day the heart in the chick beats regularly, about 40 times per minute. 

 At the anterior (aortic) extremity of the heart, the aorta originates from 

 the bulbus aortae; it bends forward, and, dividing into two arches 

 (primitive aortas), it curves beneath the brain-vesicles and descends 

 posteriorly in front of the primitive vertebrae. Both primitive aortas 

 originally terminate blind at the caudal extremity of the embryo. 

 Opposite the omphalomesenteric duct each primitive aorta in chicks 

 gives off one, in mammals several (in the dog 4 or 5) omphalomesenteric 

 arteries (Fig. 376, VI, Ao) which divide within the mesoblast upon the 

 yolk-sac, or the umbilical vesicle, into a rich network of vessels. These 

 unite and, passing backward (in birds arising from the terminal sinus 

 of the subsequent terminal vein of the area vasculosa), form omphalomes- 

 enteric veins (Vo), which ascend on the duct and empty into the two 

 venous trunks of the heart by means of two branches. 



Thus the first or primitive circulation is completed. Its pur- 

 pose is to convey nutritive material for growth and oxygen to the 

 embryo. The latter, in birds, passes through the porous shell of the 

 egg from the air ; the first is supplied by the yolk-sac until the end of 

 the incubation. In mammals both are supplied to the ovum from the 

 vessels of the uterine mucosa. In birds, on account of the consumption 



