370 
MESSRS. F. M. BALFOUR AND W. N. PARKER OK THE 
We could detect no true neurenteric canal, although a more or less complete fusion 
of the germinal layers at the tail end of the embryo may still be traced. 
During this stage the protoplasm of the notochordal cells, which in the last stage 
formed a kind of axial rod in the centre of the notochord, begins to spread outwards 
toward the sheath of the notochord. 
Eighth day after impregnation .-—The external form of the embryo (Plate 21, fig. 9) 
shows a great advance upon the stage last figured. Both head and body are much more 
compressed laterally and raised from the yolk, and the head end is folded off for some 
distance. The optic vesicles are much less prominent externally. A commencing 
opercular fold is distinctly seen. Our figure of this stage is not, however, so satisfactory 
as we could wish. 
A thickening of the nervous layer of the external epiblast which will form the lens 
(Plate 23, fig. 35, l.) is more marked than in the last stage, and presses against the 
slightly concave exterior wall of the optic vesicle (op.). The latter has now a large 
cavity, and its stalk is considerably narrowed. 
The auditory vesicles (Plate 23, fig. 36, au .) are closed, appearing as hollow sacs 
one on each side of the brain, and are no longer attached to the epiblast. 
The anterior opening of the segmental duct can be plainly seen close behind the 
head. The lumen of the duct is considerably larger. 
The two vertebral portions of the mesoblast are now separated by a considerable 
space from the epiblast on one side and from the notochord on the other, and the cells 
composing them have become considerably elongated from side to side (Plate 23, 
fig. 37, ms.). 
In some sections the aorta can be seen (Plate 23, fig. 37, ao.) lying close under the sub- 
notochordal rod, between it and the hypoblast, and on either side of it a slightly 
larger cardinal vein ( cd.v .). 
The protoplasm of the notochord has now again retreated towards the centre, 
showing a clear space all round. This is most marked in the region of the trunk 
(Plate 23, fig. 37). The sub-notochordal rod ( s.nc .) lies close under it. 
A completely closed fore-gut, lined by thickened hypoblast, extends about as far 
back as the auditory sacs (Plate 23, figs. 35 and 36, al). In the trunk the hypoblast, 
which will form the walls of the alimentary tract, is separated from the notochord by a 
considerable interval. 
Ninth day after impregnation: External characters .—Very considerable changes 
have taken place in the external characters of the embryo. It is about 8 millims. in 
length, and has assumed a completely piscine form. The tail especially has grown in 
length, and is greatly flattened from side to side : it is wholly detached from the yolk, 
and bends round towards the head, usually with its left side in contact with the yolk. 
It is provided with well-developed dorsal and ventral fin-folds, which meet each other 
round the end of the tail, the tail fin so formed being nearly symmetrical. The head is 
not nearly so much folded off from the yolk as the tail. At its front end is placed a 
