502 URO-GENITAL SYSTEM. 



Yet, the existence and functions of the umbilical vesicle, 

 or yolk sac, do not continue for any great length of time in 

 the mammalia. The nutritive pabulum enclosed by it is not 

 large ; even at the fourth week the umbilical vesicle begins 

 to atrophy, and towards the fifth only a trace of it remains 

 (Fig. 142). In the ovipara, however (especially in birds), 

 the umbilical vesicle lasts much longer, and plays a more 

 important part in the nutrition of the chick ; it contains the 

 yellow substance, a provision which is sufficient for the devel- 

 opment of the chick in the egg, and feeds it even after the 

 chicken is hatched, but at that time it is enclosed inside the 

 abdominal cavity, until the chicken is able to feed himself. 



Amnion. As soon as the umbilical vesicle and the body 

 of the embryo have been completely separated by the stran- 

 gulation that we have already studied (intestinal and cuta- 

 neous umbilicus), the distinction between the three layers of 

 the blastoderm becomes more and more complete, and the 

 external one gives rise to a particular formation, the amnion 

 and secondary chorion. In fact, as soon as the cutaneous 

 umbilicus is formed, and at the same point, the external fold 

 (cutaneous) of the blastoderm extends in such a way as to 

 surround the embryo, and forms two lateral layers which 

 tend to unite at its dorsal region, forming at the two extrem- 

 ities two hood-like folds (cephalic and caudal, Fig. 137, 6, 7) 

 which cover its caudal and cephalic portions. Only the 

 middle portion of the back of the embryo remains uncovered, 

 but soon these folds and layers by process of development 

 unite (Fig. 137, 8), until the only opening (amniotic umbili- 

 cus, Fig. 139, 8) is circumscribed and completely closed. 

 fVoin this time the embryo is enclosed in a cavity, amniotic 

 cavity (Fig. 139, a), in which it is suspended in the ambient 

 fluid, amniotic fluid, given out from the walls which form 

 this cavity. 



The internal surface of the amniotic cavity is formed by 

 that entire portion of the external layer of the blastoderm, 

 which has been isolated from the rest of this fold by the suc- 

 cessive hood-like covering of the embryo and the union of 

 the amniotic umbilicus. This surface is covered by an epi- 

 thelial layer given off from a layer of embryonic connective 

 tissue (from the middle fold), in which smooth muscular 

 fibres may be seen (Fig. 140, 141, dark and dotted lines). 

 On account of this formation the rest of the external fold of 

 the blastoderm is henceforward completely isolated from the 

 body of the embryo, and forms an extended envelope sub- 



