664 MANUAL OF PHYSIOLOGY. 



rived from- the mesoblast, is of a tougher character than the inner 

 epithelial layer, and it is said to possess muscular fibre and to be 

 capable of rhythmical contractions. 



(2) The Yolk Sac is that part of the blastoderm which grows 

 and envelops the yolk, which previously was only surrounded by 

 the vitelline membrane. After the mesoblast has split into two 

 layers, the splanchnopleure becomes bent inwards at a point some 

 distance from its origin, carrying with it the hypoblast. By this 

 curve an upper constricted canal is differentiated from the large 

 lower cavity. This upper canal becomes eventually the alimen- 

 tary tract, the lower cavity the yolk-sac, whilst the constricted 

 portion leading from the one to the other is the canal leading 

 from the intestine to the yolk, called the ductus vitello-intestinalis. 



At first the splanchnopleure incloses only the upper part of the 

 yolk, but as development proceeds it grows around, and at last 

 completely encircles it. The yolk-sac is thus derived from the 

 splanchnopleural layer of the mesoblast, and its lining hypoblast. 



The yolk 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 shrivelled protrusion 

 from the intestine, lying in the umbilical cord. 



The importance of the yolk-sac 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 yolk 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 mamma- 

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

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

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

 outwards of a portion of the splanchnopleure forming the wall of 

 the primitive intestine. It is lined by hypoblast, and projects 



