184 General Notes. [ February, 
terminate towards the dorsal pole of the ovum instead of the ven- 
tral, as in Fig. B. 
In the ovum of the guinea-pig the obliteration of the umbilical 
vesicle y is carried still farther than in Fig. F, because the hypo- 
blastic layer “y’, next to the layer o/, is absent, and the hypoblast 
lying just under the embryo is brought into immediate contact 
with the layer o/, thus giving rise to the illusion that a complete 
inversion of the primary embryonic layers has occurred. I say 
illusion, because there has been no actual inversion of the primary 
layers, for the latter have been merely shoved to the opposite 
pole of the eggs into contact with the layer o/, where embryonic 
development has proceeded in the normal way, being modified 
only by the displacement which the germinal area has suffered in 
relation to the other essential parts of the ovum. It is as if the 
germinal pole of the blastodermic vesicle had become concave 
instead of convex, and collapsed inwards against the inside of its 
lower pole, the walls of which consist of the hypoblast of the 
inferio? pole of the umbilical vesicle—mesenteron, and the 
outer layer. 
he difficulties which Balfour speaks of have, I hope, been 
satisfactorily cleared away by what has been said above, and a 
rational and connected hypothesis as to the genesis of the amnion 
firmly established. I am aware that many objections may be 
urged against the views here propounded, but I cannot think that 
ny other view of the case will so satisfactorily reconcile and 
coordinate the facts involved. To those who takea philosophical 
view of such subjects, it will be obvious that the deductions here 
reached give but little countenance to the idea that amniotic char- 
acters can be always profitably used in taxonomy, at least, not 
until the forces which have led to their development are better 
understood. On the theory of the development of development, 
the extreme modification of the amnion of some of the Rodentia 
would cause the latter to take higher rank than the Primates, 
because, as shown in Fig. F, the primary amniotic cavity becomes 
divided, and a relatively large false amniotic cavity f remains just 
under the suspensor s, and shut off from the true amniotic cavity 
ac by the intervening serous envelope se, the coelomic space ¢, an 
the somatopleural roof of ac. Such reasoning, however, is obvi- 
ously not legitimate in the light of the above mechanical hypoth- 
esis of the genesis of the amnion me 
To briefly summarize, we find that the first traces of amniotic 
folds met with in the embryos of the lower types of Chordata are 
_ caused by the resistance from without offered to the growth 
the embryo by a rigid zona radiata. In such types the amniotic 
folds are transitory, and disappear at the time the zona is ru tured. 
After a larger yolk has been acquired the embryo undergoes 4 
longer period of intra-oval development, so that the period of the 
persistence of the amniotic folds, produced as before, is prolon 
MR 
