350 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
two other embryonic structures which are peculiarly characteristic, 
the allantois and the amnion, to which reference has been made before. 
The amnion arises as a fold of the somatic wall of the ccelom in 
front of and on either side of the embryo. These folds extend upward 
and then inward until they finally meet above the embryo, thus en- 
closing it in an amniotic cavity. The folds fuse in the middle line and 
then the two sides break through so that above the wall of the amniotic 
cavity—the true amnion—there is a second cavity directly continuous 
with the ccelom, and this is bounded externally by the rest of the 
amniotic fold, this part being called the serosa or false amnion. This 
lies immediately beneath the vitelline membrane of the egg or its 
equivalent, to which many different names have been given. 
Little is known as to the phylogeny of the amnion, a structure without parallel 
in the animal kingdom except in the scorpions, where one is formed in the same 
way. Of course there is no genetic connexion between the two. It has been sug- 
gested that in both groups there is a tendency for the embryo to sink into the yolk 
and that the amnion is to prevent its being completely covered with this substance. 
The homologue of the allantois is found in the urinary bladder of 
theamphibia. Itisan outgrowth from the hinder end of the alimentary 
tract and consists of a lining of entoderm, covered externally with the 
splanchnic layer of the mesoderm—is purely splanchnopleuric—and 
projects into the ceelom. In its outgrowth it carries with it branches 
of the hypogastric blood-vessels, now known as the allantoic arteries 
and veins (usually but a single vein). As it develops, the distal 
end of the allantois swells into a large vesicle, connected with the 
digestive tract by a slender stalk. The vesicle extends into the ccelom 
between the amnion and serosa and soon fuses with the serosa. The 
terminal sac flattens and gradually extends until it encloses the whole 
embryo and amniotic sac. 
In the sauropsida the allantois (and serosa) comes eventually to 
lie just beneath the shell, and as the latter is porous and the allantois 
is very vascular, the latter is in position to act as the respiratory 
apparatus of the growing young. The cavity of the allantois, con- 
nected by its stalk with the cloacal region, serves as the reservoir for 
the urine. 
While the embryo is increasing in other respects, the side walls of 
the body gradually close in ventral to the embryo until they reach the 
stalks of the yolk sac and the allantois. In this way these structures 
come to be connected with the body by a narrow cord, called in mam- 
