of the Fishery Board for Scotland. 
313 
this round vesicle is shown as single, lying underneath the posterior end 
of the notochord, but observations on the living egg showed in more than 
one instance tlie appearance of one large and several smaller vesicles. 
These were situated at the very posterior of the embryo, and the smaller 
vesicles disappeared, while the larger vesicle became enlarged and assumed 
a more anterior position, persisting as Kupffer's vesicle. The appearance 
of the vesicle was manifest not till the blastopore was greatly reduced in 
size, about one day before the disappearance of the blastopore. In the 
cases where more than one vesicle was seen, these vesicles appeared more 
posteriorly than when only one vesicle was seen, but this may be due to 
the multi- vesicular stage being an earlier stage than the uni-vesicular state. 
1 noticed in the living egg something similar to what M'Intosh and 
Prince"'^ state as to the growth of the embryonic area over the vesicles. 
The vesicles when first seen were posterior to the caudal portion of the 
embryo, but a short time afterwards the single vesicle which resulted from 
this multi-vesicular condition was covered by the advancing tail of the 
embryo. In no case did I find these secondary vesicles to persist for 
above an hour or so. The growth of Kupffer's vesicle was proportionate 
to the decrease in size of the blastopore. It persisted for nearly two days 
from its first appearance to its disappearance; and is seen in figs. 8, 9, 10, 
and 10a (K.v.). It is last seen as a projecting vesicle under the caudal end 
of the embryo (fig. \0a.), when the blastopore (figs. 10 and 10a, bl.), has 
just closed ; but I have never seen it so far forward as M'Intosh and 
Prince figure it on PI. V. fig. 6. While the blastopore is rudely rectangular 
in fig. 8, it is elliptical in fig. 9, and is shown as a spot in fig. 10. Simul- 
taneously with the disappearance of the blastopore the embryo is more 
constricted off from the tissue which has epibolically overspread the yolk- 
niass. 
By the time that Kupffer's vesicle has appeared four or five pairs of pro- 
to vertebrae (pr.) have arisen (fig. 8), and this number is gradually increased 
both head and tailwards (figs. 8 and 10). The proto vertebrae were first 
noticed when the embryo was 5 days hours old, and on the ninth day 
onwards the myotomes of the trunk were formed (figs. 12, et seq., my). 
The pigment on the trunk is very well marked along the edge of the 
myotomes, being either black or lemon yellow. 
Nervous St/stem and Sense Organs. — The divisions of the brain are 
indicated as early as the fifth day (fig. 7, f.b., m.b., h.b.), and in fig. 9 is 
the first external indication of the cavity of the brain. After the seventh 
day (fig. 10) the constriction of the mid brain from the hind brain is shown. 
The mid brain is somewhat pyriform in shape, and posteriorly (fig. 11) 
arches over the inner side of the eyes. The hind brain is shortened and 
broadened, and the opening of the fourth ventricle is very marked. From 
the stage represented at fig. 10 onwards the continuation of the brain 
cavity and the central canal of the neurochord (ne., fig. 11) is seen. The 
pineal gland {pn., fig. 15) is seen in side view of the embryo just before 
and subsequent to the period of hatching. In the earlier stages of the 
embryonic history the different regions of the brain lie in the same plane, 
but as development proceeds the mid brain arches over both the forebrain 
and the cerebellum. Figs. 10-21 illustrate this displacement of the relative 
parts of the brain. When the yolk is absorbed (fig. 21) the cerebellum 
is almost completely covered dorsally by the mid brain, while the line 
drawn along the surface of the mid brain anteriorly, of the pineal gland, 
and of the fore brain, is almost at right angles to that drawn on the dorsal 
surface of the mid brain and^cerebellum. This shows the great curvature 
Ut supra. 
