DE VELOPMENT OF REPTILES. 589 



blastoderm behind the future embryonic region. The embryo develops 

 in front of the primitive streak, and one of the first signs of its develop- 

 ment is the formation of a primitive or medullary groove in a line with 

 the primitive streak. As development proceeds, folds appear around 

 the embryo, constricting it off from the subjacent yolk or yolk-sac. 



It is with Reptiles that the series of higher Vertebrates or Amniota 

 begins. It is here that the fcetal 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 

 upwards, 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. 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 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 Cyrfodics among Lizards, the vascular yolk-sac is separated from 



