EARLY STAGES IN THE DEVELOPMENT OF VERTEBRATES. 215 
normal with vertebrates, consists of a single layer of 
columnar cells. 
In the next stage (p 11) the formation of the alimentary 
canal (al) has commenced, but it is to be observed that 
there is in this case no true involution. 
As an accompaniment to the encroachment upon the seg- 
mentation cavity, which was a feature of the last stage, the 
cells to form the walls of the alimentary canal have come to 
occupy their final position during segmentation and without 
the intermediation of an involution, and traces only of the 
involution, are to be found in (1) a split in the lower layer 
cells which passes along the line separating the small and 
the large lower layer cells ; and (2) in the epiblast becoming 
continuous with the hypoblast on the dorsal side of the 
mouth of this split. It is even possible that at this point a 
few cells (though certainly only a very small number) of 
those marked yellow in p 1 become involuted. This point 
in this, as in all other cases, is the tail end of the embryo. 
The other features of this stage are as follows :—(1) The seg- 
mentation cavity has become smaller and less conspicuous 
than it was. (2) The epiblast cells have begun to grow round 
the yolk even in a more conspicuous manner than they didin 
the frog, and are accompanied by a layer of mesoblast cells 
which again becomes thickened at its edge. ‘The mesoblast 
cells in the region of the body are formed in the same way 
as before, viz. by the separation of a layer to form the 
epithelium of the alimentary canal, the other cells remaining 
as mesoblast; and as in the frog, or in a more conspicuous 
manner, we find that the dorsal surface only of the alimentary 
cavity has a wall formed of a distinct layer of cells, but on 
the ventral side the cavity is at first closed in by the large 
spheres of the yolk only. The formation of the alimentary 
canal by a split and not by an involution is exactly what 
Bambeke finds in Pelobates. 
The next stage, p 111, is about an equivalent age to c 111 
in the frog. It exhibits the same connection between the 
neural and the alimentary canals as was found there. 
The alimentary canal is beginning to become closed in 
below, and this occurs near the two ends earlier than in the 
middle. ‘The cells to form the ventral wall are derived from 
the large yolk-cells. The non-formation of the ventral wall 
of the alimentary canal so soon in the middle as at the ends 
is an early trace of the umbilical canal found in Birds and 
Selachians, by which the alimentary tract is placed in com- 
munication with the yolk-sac. The segmentation cavity 
has by this stage completely vanished, and the epiblast with 
