THE FOllMATlON OF THE LAYERS IN AMPHIOXUS. 333 
yolk-sac was the so-called pro-amnion, which lay in front of 
and beneath the head, and here the flexure took place. 'I'he 
inere fact that in Mammalia there is here no meeting of the 
lateral sheets of mesoderm, so that the diploblastic head 
envelope is called “pro-amnion,” whereas in reptiles the lateral 
sheets of mesoderm early meet beneath the head, and so a 
“ true head fold of the amnion ” is developed, seems to me to 
be a secondary affair. This bending tended, just as a ship 
sinking by the head tends to lift her stern, to raise the tail 
region and so bring the bladder-like outgrowth of the gut — 
already present in Amphibia — near the surface of the egg', 
and so to increase its chances of getting oxygen. In this the 
foundation was laid for the modification of the allantois into 
a breathing organ and for the corresponding development of 
the tail-fold of the amnion, the two developing, as Balfour 
long ago showed, together, since the tail-fold of the 
amnion contains the extension of the bladder or allantois. 
Hubreclit scornfully asks if the pro-arnnion was developed 
to contain the head, Avhy there is none in the human embryo ? 
The answer is easy; in the human embryo the embryo itself 
is as long as the yolk-sac from the beginning, the yolk-sac 
being in this case a vestigial organ which has suffered great 
reduction in size, and hence when the embryo elongates there 
is no yolk-sac for it to plunge its head into. Space, however, 
will not permit us to pursue this subject further. To an 
author like Hubreclit, who finds no difficulty in supposing 
that the oviparous mode of development in Echnida is secon- 
darily derived from a placental method of development, such 
as is found in the rabbit, no change is so unlikely as to seem 
impossible. 
Apart from Hubrecht’s desire to prove that the ancestors 
of Mammals never had yolky eggs, the main result of his 
paper is to advocate the view that Vertebrates have developed 
from an Actiuian ancestor. The protochordal wedge repre- 
sents, according to him, the old stomodaeum which opened 
into the true endodermal gut below. The old mouth was 
originally surrounded by a nerve ring, but it became closed 
