EMBRYO AND EMBRYONIC MEMBRANES 225 



bation over the embryonic pole and gradually slips down over 

 the yolk, and is finally gathered together in the albumen-sac. 



(4) The allantois also serves as a reservoir for the secretions 

 of the mesonephros, and subsequently the permanent kidney, 

 which reach it by way of the cloaca and neck of the allantois. 

 The fluid part of the embryonic urine is absorbed, but the con- 

 tained salts are deposited in the walls and cavity of the allantois. 

 If the connection between the Wolffian ducts and cloaca be inter- 

 rupted, the former become enormously extended by the secre- 

 tions of the mesonephros. 



The Yolk-sac. The yolk-sac is established in the manner 

 already described; it is constituted by the extra-embryonic 

 splanchnopleure, and is permanently united to the intestine by 

 the yolk-stalk. A narrow lumen remains in the stalk of the 

 yolk-sac throughout, and even after, incubation, but the yolk 

 does not seem to pass through it into the intestinal cavity. The 

 walls of the yolk-sac, excepting the part derived from the pellucid 

 area, are lined with a special glandular and absorbing epithelium, 

 which digests and absorbs the yolk and passes it into the vitel- 

 line circulation, through which it enters the hepatic portal circu- 

 lation and comes under the influence of the hepatic cells. The 

 yolk-sac is thus the primary organ of nutrition of the embryo, 

 and it becomes highly elaborated for the performance of this 

 function. Contrary to the statements found in many text-books, 

 it does not reach its maximum development until the end of 

 incubation. Throughout incubation it steadily increases in 

 complexity and efficiency so as to provide for the extremely 

 rapid growth of the embryo. 



The functions of the yolk-sac manifestly require a large sur- 

 face area, which is provided for by foldings of the wall projecting 

 into the yolk. At the height of its development the inner surface 

 of the yolk-sac is covered with numerous folds or septa projecting 

 into the yolk, which are highest at the equator and decrease in 

 both directions away from the equator. In general, these folds 

 follow the direction of the main arteries, i.e., they run in a 

 meridional direction, repeatedly bifurcating distally (Fig. 132). 

 Moreover, each one is perforated by numerous stomata, and the 

 yolk-sac epithelium covers all free surfaces, and a capillary net- 

 work is found in every part. So far do they project into the 

 interior towards the close of incubation, that those of opposite 



