DEVELOPMENT OF REPTILES. 643 



It is here that the foetal membranes known as amnion and .allantois 

 are first formed. 



(a) The Amnion. At an early stage in development the head end of 

 the embryo seems to sink into the subjacent yolk. A semilunar fold of 

 the blastoderm, including epiblast and mesoblast, rises up in front. 

 Similar folds appear laterally. All the folds increase in size, arch up- 

 wards, and unite above, forming a dome over the embryo. Each of 

 these folds is double ; the inner limbs unite to form " the true amnion " ; 

 the outer limbs unite to form "the false amnion," "serous membrane," 

 or subzonal membrane. The cavity bounded by the true amnion 

 contains an amniotic fluid bathing the outer surface of the embryo ; 

 the cavity between the true and the false amnion is lined by mesoblast, 

 and is continuous with the pleuro-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 allantois 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. 



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 "SL 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 Mustelus 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 Cydodus 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." (Sec 

 A. C. Haddon's Embryology. ) 



