624 Proceedings of Royal Society of Edinburgh. [sess. 
the mass of cells underlying the primitive grooves. The noto- 
chords gradually come nearer together from before backwards, but 
never come into contact. 
In the four cases cited above the notochords were separate 
throughout except in Kaestner’s, where they were united at their 
posterior ends. 
Mesoblast . — In the region of the primitive groove proper (fig. 
14) the mesoblast extends outwards from the axial mass of cells as 
two lateral sheets in a typical manner. Traced forwards, the 
groove gradually dies out, and we have a flat ectodermic plate 
(fig. 13), continuous with which is a broad axial mass of indifferent 
cells, somewhat loosely arranged, from which the lateral plates of 
mesoblast extend. A few sections further forward two slight 
grooves indent the ectodermic plate, and when traced forwards, 
it is found that one of these deepens, and from its apex there 
extends a keel-like mass of cells continuous with the mesoblast 
on each side, while the other is only a slight dip in the ecto- 
dermic plate (fig. 12). Further forward, however, this dip 
deepens into a well-marked groove, beneath which is a similar 
keel-like mass of cells, so that now there are two lateral sheets of 
mesoblast, and an axial mass between the two keel-like thicken- 
ings (fig. 11). These gradually become marked off from the 
mesoblast (fig. 10), and then from the epiblast (fig. 9) forming the 
enlarged posterior ends of the notochords, while the grooves 
become the two neural furrows to be presently described. Be- 
tween the two notochords lie the azygos primitive segments. In 
some sections (fig. 7) they are simple and have a single cavity, 
while in others (fig. 8) they are double, and have two cavities. 
Neural Canal. — Fig. 1 is the 58th of the series ; from this to the 
94th (fig. 3) the medullary canals of the components are quite 
independent, and the medullary folds meet one another along 
their whole length, but do not fuse. At the level of the 84th 
section the mesial folds become enlarged, and two processes grow 
from them towards the middle line, until in the 94th (fig. 3) they 
meet and fuse. These processes project above the level of the 
somatopleure, so that a space is left between that membrane and 
the fused mesial folds. The point where the separate neural 
plates collide lies in front of the heart anlage at the anterior end 
