220 Ff. M. BALFOUR. 
The features of the next stage are the necessary con- 
sequences of those of the last. 
The ventral wall of the alimentary canal is entirely 
formed by a folding-in of the sheet of hypoblast. 
The more rapid folding-in at the head still indicates the 
previous more vigorous growth there, otherwise there is 
very little difference between the forms of the fold at the 
head and tail. ‘he alimentary canal does not of course, at 
this or any period, communicate with the neural tube, since 
the epiblast and hypoblast are never continuous. ‘The other 
features, such as the growth of the epiblast round the 
yolk-sac, are merely continuations of what took place in the 
last stage. 
In the development of a yolk-sac as a distinct appendage, 
and its absorption within the body, at a later period, the 
bird fundamentally resembles the dog fish. 
Although there are some difficulties in deriving the type 
of development exhibited by the Bird directly from that of 
the Selachian, it is not very difficult to do so directly from 
Amphioxus. Were the alimentary involution to remain 
symmetrical as in Amphioxus, and the yolk-containing part 
of the egg to assume the proportions it does in the Bird, we 
should obtain a mode of development which would not be 
very dissimilar to that of the Bird. The epiblast would 
necessarily overgrow the yolk uniformly on all sides and not 
in the unsymmetrical fashion of the Selachian egg. A con- 
firmation of this view might perhaps be sought for in the 
complete difference between the types of circulation of the 
yolk-sac in Birds and Selachians; but this is not so im- 
portant as might at first sight appear, since it is not from 
the Selachian egg but from some Batrachian that it would 
be necessary to derive the Reptiles’ and Birds’ eggs. 
If this view of the bird’s egg be correct, we are compelled 
to suppose that the line of ancestors of birds and reptiles did 
not include amongst them the Selachiansand the Batrachians, 
or at any rate Selachians and Batrachians which develope on 
the type we now find. 
The careful investigation of the development of some 
reptiles might very probably throw light upon this im- 
portant point. In the meantime it is better to assume that 
the type of development of birds is to be derived from that 
of the Frog and Selachians. 
Summary.—If the views expressed in this paper are 
correct, all the modes of development found in the higher 
vertebrates are to be looked upon as modifications of that of 
Amphioxus, It is, however, rather an interesting question 
