DEVELOPMENT OF GERMINAL LAYERS IN MAMMALS. 443 
cardial portion of the celom. It seems probable that renewed 
observations on the development of the cat, aided by longi- 
tudinal sections of the ovum, will remove the distinction which 
at present is supposed to exist between the pro-amnion forma- 
tion in the cat and other Vertebrates. ’ 
It has already been shown that the pro-amniotic area in the 
rat and the mouse is very small when it first becomes defined 
(figs. 15 and 15 C, P]. XXIV). It increases for a time, but is 
obliterated by the extension of the mesoblast between its layers 
before the cephalic curvature appears. It is carried upwards 
in front of the embryonic area not by the sinking of the 
cephalic extremity of the embryo into the yolk-sac—an im- 
possible occurrence in the rat and the mouse,—but by a folding 
of the yolk-sac wall which takes place in front of the embryonic 
area, synchronously with the formation of the mesoblast and 
the celom round the margin of the epiblast. 
The amnion folds in the rat and the mouse are completed 
after the formation of the coelom, and as the ccelom is most ex- 
tensive posteriorly the tail amnion fold is most developed. But 
all the folds are from the first continuous with each other, and 
the lateral and cephalic folds are mere extensions of the tail 
fold. This is the case not only when they are caused by the 
solid mesoblastic ridge, but also when the ridge becomes 
hollowed by the formation of the ceelom. The lateral folds 
appear before the mesoblast has become bilaminar, not after, 
and the lamin of the lateral folds are not at first separate 
from the cavity of the tail fold, as Selenka asserts. They 
therefore correspond closely with the amnion folds of the 
rabbit, which also commence at the caudal end and extend 
round the sides of the embryonic area, meeting eventually some 
distance in front of it where the cephalic fold is formed. In 
fact, the whole of the amnion is formed by the extension of the 
caudal fold which embraces the embryonic area, passing 
gradually through the crescentic to the circular or oval form, 
its margin afterwards becoming contracted and finally ob- 
literated at the amnion navel, the fold at the same time being 
converted into a hood. In the rat, the mouse, the rabbit, the 
