218 EMBRYOLOGY. 



On the fourth day the urinary sac is so enlarged that it can 

 no longer find room in the embryonic part of the body-cavity, and 

 therefore forces itself into the extra-embryonic portion of it between 

 the intestinal and dermal portions of the umbilical stalk (Plate I., 

 fig. 5 aT). Here it comes into the space between the yolk-sac (ds) and 

 amnion (A) ; then it comes in contact with the inner surface of the 

 serosa (S), and spreads out under it for a considerable distance over 

 the right side of the embryonic body. 



In regard to the subsequent fate of the embryonic membranes in the 

 Chick, it is to be noticed that up to the middle of incubation, i.e., up 

 to about the eleventh day, they continue to develop in a progressive 

 direction, but that from this time onward certain regressive processes 

 commence, which later become more and more apparent. 



In the first period (fifth to eleventh day) the' following changes 

 are effected in the yolk-sac, the amnion, the allantois, etc. The 

 vascular area spreads out, in the manner before described, over a 

 greater area in the wall of the yolk-sac, which still retains a 

 considerable size. On the seventh day it covers about two-thirds 

 (Plate I., fig. 5), and on the tenth three-fourths of the yolk-sac. At 

 the same time the marginal vein becomes indistinct, and the sharp 

 separation from the non-vascular portion ceases. 



The contents of the yolk-sac have become fluid by chemical 

 changes of the yolk-mass. The serosa (S) is raised from its surface 

 as far as the vascular area has extended, owing to the enlargement 

 of the extra-embryonic body-cavity. At the same time the allantois 

 (Plate I., fig. 5 al) has grown into the intermediate space. This has 

 enlarged so much by the tenth day that it leaves uncovered only a 

 small portion of the yolk-sac and amnion. It has lost still more 

 of its sac-like character ; for between its outer layer, which almost 

 everywhere is closely applied to the inner surface of the serosa, and 

 its inner layer, adjoining the amnion and yolk-sac, there is found only 

 an insignificant intermediate space filled with urine. 



The allantois, moreover, has by this time become a very vascular 

 organ and is nourished by the umbilical vessels, which will engage 

 our attention in a subsequent chapter devoted to the vascular system. 

 The network of blood-vessels is densest in its outer layer, which 

 spreads out at the surface of the egg ; it serves to maintain here the 

 processes of embryonic respiration, since carbonic acid is given off from 

 the superficially circulating blood and oxygen is taken up. The latter 



