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, z.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 
