CROCODILES, ALLIGATORS, G A VIALS. 591 



peritoneal or body cavity of the embryo. The amniotic folds extend 

 not only over the embryo, but ventrally around the yolk sac which they 

 completely invest. 



(b) The Allantois. While the amnion is being formed, a sac grows 

 out from the hind end of the embryonic gut. This is the ajlantois, lined 

 internally by hypoblast, externally by mesoblast. It rapidly insinuates 

 itself between the two limbs of the amnion, eventually surrounding both 

 embryo and yolk sac. 



The amnion is a protective membrane, forming a kind of water bag 

 around the embryo. It may be due in part to the embryo sinking 

 into the yolk sac by its own weight. 



The allantoic sac is vascular, and has respiratory and perhaps also 

 some yolk absorbing functions. It seems to be homologous with the 

 outgrowth which forms the cloacal bladder of Amphibians ; it has been 

 called "a precociously developed urinary bladder." 



Before the amnion is developed, the heavy head end of the embryo 

 has already sunk into a depression (in Lizards, Chelonians, Birds (?) 

 and Mammals), and is surrounded by a modification of the head fold 

 termed the pro-amnion. This does not include any mesoblast, and is 

 afterwards replaced by the amnion. 



Some Peculiarities in Chelonians. Mitsukuri has recently investigated 

 the development of the foetal membranes in Chelonians (Clemmys and 

 Trionyx], and has demonstrated some interesting peculiarities. 



The amnion has at first the nature of a pro-amnion, consisting in the 

 region of the sunken head of epiblast and hypoblast, and in the dorsal 

 region of epiblast alone, being as yet non-mesoblastic. The ccelomic 

 cavities of the amniotic folds are not united with each other dorsally in the 

 usual fashion ; a connection between the " true amnion " and the " serous 

 membrane" separates the cavities to the very end of the development. 

 The anterior and lateral amniotic folds are continued backward beyond 

 the posterior end of the embryo, as a long tube connecting the amniotic 

 sac with the exterior. This tube perhaps conveys nutritive matter from 

 the albumen into the amniotic cavity. In Clemmys, a process from 

 the foetal membranes projects into a small persistent mass of albumen, 

 and seems to absorb nutritive particles. 



Hints of a Placenta before Mammals. As will be explained after- 

 wards, the placenta, which characterises most Mammals, is an organic 

 connection between mother and unborn young. Its embryonic part is 

 chiefly formed from a union of the serous or sub-zonal membrane and 

 the allantois, but in some cases the yolk sac and the sub-zonal membrane 

 form a provisional placenta. The placenta establishes a vital union 

 between the embryo and the mother. 



Now it is interesting to notice, that there are some hints of placenta 

 connection in animals which are much lower than Mammals. In 

 some species of Mustelus and Carcharids, there is a connection 

 between the yolk sac and the wall of the uterus ; in the Teleostean 

 Anabteps, the yolk sac has small absorbing outgrowths or villi ; in 

 Trachydosaurus and Cyclodus among Lizards, the vascular yolk sac is 

 separated from the wall of the uterus " only by the porous and friable 

 rudiment of the egg shell ; " in Clemmys among Chelonians, there is, 

 as above described, an absorbing protrusion of the foetal membranes. 



