EARLY STAGES IN THE DEVELOPMENT OF VERTEBRATES. 219 
and extends a short way beyond the edge of the lower layer 
cells. 
In the next stage the more important departures from the 
previous type of development become visible. 
The epiblast spreads uniformly over the yolk-sac and not 
on the one side only as in the former eggs. 
This is due to the embryo (indicated in Fit by a 
thickening of the cells) lying in the centre and not at the 
edge of the blastoderm. <A necessary consequence of this is, 
that the epiblast does not, as in the previous cases, become 
continuous with the hypoblast at the tail end of the embryo. 
This continuity, being of no functional importance, could 
easily be dispensed with, and the central position of the 
embryo may perhaps be explained by supposing the process, 
by which in the Selachian egg the blastopore ceases to corre- 
spond in position with the opening of the alimentary slit 
or anus of Rusconi (vide E’), to occur quite early during seg- 
mentation instead of at a late period of development. For 
the possibility of such a change in the date of formation, the 
early appearance of the nervous and epidermic layers in 
the Frog affords a parallel. 
The epiblast in its growth round the yolk is only partially 
accompanied by mesoblast, which, however, is thickened at 
its extreme edge as in the frog. Owing to the epiblast not 
becoming continuous with the hypoblast at the tail end of 
the embryo, the alimentary slit is not open to the exterior. 
The hypoblast is formed by some of the lower layer cells 
becoming distinguished as a separate layer; the remainder 
of the lower layer cells become the mesoblast. 
The formation of the mesoblast and hypoblast out of the 
lower layer cells has been accepted for the Bird by most 
observers, but has been disputed by several, and recently 
by Kolliker. These have supposed that the mesoblast is 
derived from. the epiblast. I feel convinced that these 
observers are in the wrong, and that the mesoblast is 
genuinely derived from the lower layer cells. 
The greater portion of the alimentary cavity consists of 
the original segmentation cavity (vide diagrams). This 
feature of the segmentation cavity of Birds sharply dis- 
tinguishes it from any segmentation cavity of other eggs, 
and renders it very doubtful whether the similarly named 
cavities of the Bird and of other vertebrates are homologous. 
On the floor of the cavity are still to be seen some of the 
formative cells, but observers have not hitherto found that 
they take any share in forming the ventral wall of the 
alimentary canal, 
