440 ARTHUR ROBINSON. 
with the pro-amnion fold of the rabbit. When first distin- 
guishable this portion of the cephalic fold is extremely short ; 
and although it increases for a time it is, in comparison with 
the same area in other animals, always relatively small. In 
connection with this area in the ova of the mouse and the rat it 
is necessary to consider van Beneden’s hypothesis of the inver- 
sion of the germinal layers. In discussing the formation of 
the pro-amnion fold in the rabbit, van Beneden has pointed out 
that it results from the sinking of the cephalic extremity of 
the embryo into the yolk-sac, which is thus gradually invagi- 
nated, and in the walls of the invagination cavity the layers 
are inverted, the epiblast being turned inwards towards the 
embryo, and the hypoblast outwards towards the chorion (8). 
In asimilar manner a pro-amnion fold is formed in Lacertilians 
(20, Taf. i, figs. 3 and 4; 49, Taf. 1, fig. 2), Chelonians (85), 
Aves (39, 48), Cheiroptera (3), Insectivora (24), and the opossum 
(46). The process is therefore very general, and van Beneden 
suggests the probability that the inversion which occurs in so 
many rodents is simply an accelerated pro-amnion formation. 
If this supposition is correct, it is evident that as the inver- 
sion of the membrane commences before the weight of the 
embryonic area can be supposed to have any effect in its 
production, it must be due to the precocious expression of an 
inherited propensity which tends to develop itself indepen- 
dently of circumstances. It is noteworthy in connection with 
this subject that the inverted portions of the wall of the ova 
which undergo typical inversion are much more extensive 
then the amniotic fold—that is, a portion of the false amnion 
is also inverted ; and thus we find that the inherited tendency — 
is not only precocious in the time of its appearance, but it also 
produces excessive results. 
Further, the pro-amniotic area is bounded posteriorly by 
the pericephalic mesoblast, and in front by the inner margin 
of the ring of the extra-embryonic mesoblast which is formed 
by the fusion of the two horns of the crescentic extension of 
the middle layer. The pro-amniotic area is therefore inti- 
mately associated with mesoblastic formation. It is an area 
