442 ARTHUR ROBINSON. 
liferation is rapid, probably as the result of an inherited 
tendency to expand rapidly over a comparatively large yolked 
ovum, its constituent parts become massed together in the 
uterine crypt into a solid rod of trophoblastic tissue (fig. 10, 
Pl. XXIII). During the increase of the rod its distal end 
pushes the epiblast before it, and thus continues still further 
invagination of the yolk-sac (figs. 11 and 12, Pl. XXIII). The 
inversion of the membranes in the rat and the mouse does not, 
therefore, seem to be due to the precocity of an inherited 
tendency, but rather to a series of modified circumstances 
which influence and change the normal processes of develop- 
ment. These circumstances are— 
1. The inclusion of the ovum in a narrow uterine crypt. 
2. The adhesion of the trophoblast to the wall of the yolk- 
sac. 
8. The adhesion of the trophoblast to the wall of the crypt, 
and its subsequent extension in the cavity of the crypt. 
4, The unequal rapidity in the increase of the ovum, and of 
the crypt cavity within which it is contained. 
The invaginated walls of the ova of rats and mice cannot be 
considered as true pro-amniotic folds. They are not formed to 
enclose the embryonic area beneath a hood-like covering, but 
are the result of modified developmental conditions, and only 
portions of them are subsequently transformed into the true 
amnion folds. 
The pro-amniotic area in the rat and the mouse corresponds 
with the pro-amniotic areas of all other Amniota except the cat, 
in which animal, according to Fleischmann’s description (12), 
the pro-amniotic area is not present in the early stages, but 
appears later when the cephalic curvature commences, and lies 
immediately in front of the rudimentary head. Therefore, 
‘whether the pericephalic mesoblast disappears or not, the pro- 
amnion and the bucco-pharyngeal membrane must be continu- 
ous structures. This is an improbable circumstance, for the 
pericephalic mesoblast in all animals becomes the wall of the 
pericardiac cavity, and Fleischmann’s figures show that the 
pro-amnion of the cat lies at the sides and in front of the peri- 
