EXTINCT REPTILES. 621 



body cavity of the embryo. The amniotic folds extend not only over 

 the embryo, but ventrally around the yolk-sac, which they completely 

 invest. 



(/>) The Allantois. While the amnion is being formed, a sac grows out 

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

 internally by hypoblast, externally by mesublast. 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. 



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 subzonal membrane and 

 the allantois, but in some cases the yolk-sac and the subzonal 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 placental 

 connection in animals which are much lower than Mammals. In 

 some species of Mnstchts and Carcharias there is a connection between 

 the yolk-sac and the wall of the uterus ; in the Teleostean Anableps 

 the yolk-sac has small absorbing outgrowths or villi ; in Trachydosaurus 

 and Cyclodiis 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 an absorbing pro- 

 trusion of the foetal membranes. In Birds also, small villi of the yolk-sac 

 absorb yolk, and others on the allantois absorb albumen." (See A. C. 

 Haddon's "Embryology.") 



Extinct Reptiles. 



The first known occurrence of fossil Reptiles is in Permian 

 strata ; in the Trias most of the orders or classes are repre- 

 sented ; while the "golden age" of the group was un- 

 doubtedly during Jurassic and Cretaceous times. 



Some of the modern Reptiles are linked by a series of 

 fine gradations to very ancient progenitors, the Crocodiles 

 of to-day lead back to those of the Trias, the New Zealand 



