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 coelom 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 coelom, 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 

 the amphibia. It is an 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 coelom. 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- 



