DEVELOPMENT OF REPTILES. 643 
It is here that the foetal membranes known as amnion and allantois 
are first formed. 
(2) 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. 
(4) The Allantocs.—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 ‘‘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 Afzste/us and Carchardas 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 Zrachydosaurus 
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 CZemmys 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.) 
