218 F. M. BALFOUR. 
consequence of the flattened form of the original alimentary 
slit which is far too wide to form the cavity of the final 
canal. In the bird whose development must next be con- 
sidered this folding-in is a still more prominent feature in 
the formation of the alimentary canal. As in the last case, 
the alimentary canal is widely open in the middle to the yolk 
at the time when its twoends areclosed below and shutoff from 
it ; still later this opening becomes very narrow and forms 
the duct of the so-called umbilical cord which places the 
yolk-sac in communication with the alimentary canal. As 
the young animal becomes larger the yolk-sac ceases to 
communicate directly with the alimentary canal, and is carried 
about by it for some time as an appendage and only at a later 
period shrivels up. 
The mesoblast is formed in a somewhat different way in 
the sharks than in other vertebrates. It becomes split off 
from the hypoblast, not in the form of a single sheet as in 
other vertebrates, but as two lateral sheets, one on each side 
of the middle line and separated from one another by a con- 
siderable interval ; whilst the notochord is derived not as in 
other vertebrates from the mesoblast, but from the hypoblast 
(vide F. M. Balfour, ‘‘ Development of Selachians,” ‘ Journal 
of Microscopical Science,’ Oct., 1874). 
Between the Selachians and the Aves there is a consider- 
able gulf, which it is more difficult satisfactorily to bridge 
over than in the previous cases; owing to this I have not 
attempted to give any intermediate stage between them. 
The first stage of the Bird (¥ 1) is very similar in many 
respects to the corresponding stage in the Selachian. The 
segmentation cavity is, however, a less well-defined forma- 
tion, and it may even be doubted whether a true segmenta- 
tion cavity, homologous with the segmentation cavity in the 
previously described egg, is present. On the floor of the 
cavity which is the case formed by the yolk are a few 
larger cells known as formative cells which, according to 
Gotte’s observations, are derived from the yolk, in a some- 
what similar manner to the cells which were formed around 
the nuclei in the Selachian egg, and which helped to form 
the ventral wall of the alimentary canal. Another point to 
be noticed is that the segmentation cavity occupies a central 
position, and not one to the side as in the Selachian. 
The yolk is proportionately quite as large as in the Sela- 
chian’s egg, but, as in that case, there can be little or no 
doubt of its being homologous with the largest of the seg- 
mentation spheres of the previous eggs. It does not undergo 
segmentation, The epiblast is composed of columnar cells, 
