MANUAL OP PHYSIOLOGY. 



The yelk is continually used up for the nutrition of the embryo, 

 and its covering shrinks in size, becoming smaller with the growth 

 of the foetus, until eventually it forms but a shriveled protrusion 

 from the intestine, lying in the umbilical cord. 



The importance of the yelk sack differs largely in mammalia and 

 birds. In man it is not highly developed, as its place is early 

 supplied by the placenta. In birds, however, it develops to a 

 much higher degree, being the seat of a special circulation, which 

 carries nourishment from the yelk to the chick. The vessels are 

 developed in the mesoblastic portion of the membrane, and are 

 called the omphalo-mesenteric vessels, which convey blood to and 

 from the primitive heart. 



(3) The Allantois, or urinary vesicle, in the chick is of import- 

 ance, as the vessels developed in it are used for respiratory pur- 

 poses, being spread out beneath the porous shell. In the mam- 

 malian embryo it is still more important, as it is the seat of the 

 circulation, which performs the chief function of the fcetal pla- 

 centa. The allantois arises at the tail of the embryo, as a bud- 

 ding outward of a portion of the splanchnopleure forming the 

 wall of the primitive intestine. It is lined by hypoblast, and 

 projects into the pleuroperitoneal cavity. As it grows away 

 from the embryo it extends between the layers of the true and 

 false amnion and approaches toward the vitelline membrane, but 

 remains connected to the intestine by a narrow tube. When it 

 reaches the periphery of the ovum, it spreads over the choriou 

 as a complete lining, and sends processes into the villi of that 

 organ. It becomes chiefly developed, however, at that part of 

 the chorion which is opposite the detidua serotina of the mother. 

 In the mesoblastic layer of the allantois blood vessels arise which 

 are connected with large trunks, proceeding from the primitive 

 aortse, called the umbilical arteries; these will, however, be 

 further described when treating of the fcetal placenta. 



As the foetus becomes developed, the part of the allantois in 

 connection with the body becomes gradually obliterated. A part 

 of it remains as the urinary bladder, and the rest forms a fibrous 

 cord, which runs from the apex of the bladder to the umbilicus, 

 and is known as the urachus. 



