EARLY STAGES IN THE DEVELOPMENT OF VERTEBRATES. 991 
whether it is possible to suppose that the original type 
was not that of Amphioxus, but of some other animal, say, 
for instance, that of the Frog, and that this varied in two 
directions,—on the one hand towards Amphioxus, in the 
reverse direction to the course of variation presupposed in 
the text; and on the other hand in the direction towards 
the Selachians as before. 
The answer to this question must in my opinion be in the 
negative. It is quite easy to conceive the food material of the 
Frog’s egg completely vanishing, but although this would en- 
tail simplifications of development and possibly even make 
segmentation uniform, there would, as far as I can see, be no 
cause why the essential features of difference between the Frog’s 
mode of development and that of Amphioxus should change. 
The asymmetrical and slit-like form of involution on the one 
side and the growth of the epiblast over the mesoblast on the 
other side, both characteristics of the present Frog’s egg, would 
still be features in the development of the simplified egg. 
In the Mammal’s ege¢ we probably have an example of a 
Reptile’s egg simplified by the disappearance of the food 
material ; and when we know more of Mammalian embry- 
ology it will be very interesting to trace out the exact manner 
in which this simplification has affected the development. It 
is also probable that the eggs of Osseous fish arefundamentally 
simplified Selachian eggs; in which case we already know 
that the diminution of food material has affected but very 
slightly the fundamental features of development. 
One common feature which appears prominently in reviewing 
the embryology of vertebrates as a whole is the derivation of 
themesoblast from the hypoblast; in other words, we find that it 
is from the layer corresponding to that which becomesinvoluted 
in Amphioxus so as as to line the alimentary cavity that the 
mesoblast is split off. 
That neither the hypoblast or mesoblast can in any sense 
be said to be derived from the epiblast is perfectly clear. 
When the egg of Amphioxus is in the blastosphere stage we 
cannot speak of either an epiblast or hypoblast. It is not 
till the involution or what is equivalent has occurred, con- 
verting the single-walled vesicle into a double-walled one, 
that we can speak of these two layers. It might seem 
scarcely necessary to insist upon this point, so clear is it 
without explanation, were it not that certain embryologists 
have made a confusion about it. 
The derivation of the mesoblast from the hypoblast is the 
more interesting, since it is not confined to the vertebrates, 
but has a very wide extension amongst the invertebrates. In 
