EMBRYOLOGY OF THE SEA BASS. 
241 
embryonic canalis centralis is then acquired (Fig. 102, PI. 0 ; Fig. 127, PI. cm). When 
the central canal is first established I am disposed to think the wall of the canal is 
everywhere but one cell thick (Fig, 90, PI. xcvm). It is difficult, though, to make 
sure of this in sections (see Fig. 94, PI. xcix). After the canal is once established the 
wall increases in thickness (Fig. 110, PI. ci), but even at the time of hatching the 
posterior end of the cord has a unicellular wall (Figs. 119 and 126, PI. cii). Likewise 
in the brain when the cavity is first established, the wall is but one or at most two 
cells thick (Figs. 95, 96, and 97, PI. xcix). 
General development of the embryonic brain, — At the time of hatching, the brain 
is not only histologically undifferentiated but is morphologically exceedingly simple. 
From the surface views alone (Figs, 146-150, Pis. cvi and cvii) almost the whole de- 
velopment may be gathered. The cerebral vesicles of the higher vertebrates, which 
make their appearance in the Trout, are not developed in the Bass ; at least I have 
never found a stage in which they were present. In Fig. 146, PI. cvi, the optic sacs 
and that part of the brain with which they communicate have been hollowed out. 
, Elsewhere the neural cord is solid. In Fig. 147, PI. cvi, the central canal and its cei'e- 
' bral continuation are established, and there is a constriction (Ht. con.) marking off the 
j anterior from the posterior part of the brain. The swollen portion of the brain 
■ directly in front of the constriction develops into the mid-brain while the portion 
behind the constriction becomes the medulla. It will be seen that at first the medulla is 
narrower than the mid- brain. In Fig, 148 the fourth ventricle is developed (section 
given in Fig. 96, PI. xoix), and in connection with the appearance of this cavity, 
j aided also by the increase in thickness of its own walls, the medulla has become much 
wider than in preceding stages, and considerably exceeds the mid brain in width. A 
! longitudinal section of this stage is given in Pig. S3, PI. xcvm. Figs. 149 and 150, 14. 
I oviT, are views of the brain from below and above, respectively, not long before hatch- 
ing, The stage drawn in Fig. 150 is somewhat tbe older, but the difference scarcely 
|| concerns the brain. The constriction between the medulla and mid-brain is a deep 
[ one, but the increase in width of these two parts of the brain involves especially the 
[ dorsal surface, as may be seen on comparing tbe two surface views, or in the sections 
, Figs. 131-133, PI. CIV, and consequently both the ey^ and the ear are overarched 
I by the medulla. Two folds, the subsequent history of which I have not followed 
I out but which would seem to form the cerebellum, appear in this stage {cer. /., Fig. 
j 150). The optic nerves come off from the ventral surface of the brain and run into 
I' the anterior parts of the optic cups. The part of the forebraiu in front of the optic 
i nerves which will develop into the hemispheres is small and undifferentiated, 
j An examination of a series of transverse sections through the brain at the time of 
J hatching will supplement this description of the embryonic brain. Pigs. 128-131, 
i!P 1. cm and civ, ai’e successive sections from behind forwards through the medulla. 
I The increase in thickness of the walls and general change of shape in comparison 
[with earlier stages is seen on referring to Fig. 96, Pi. xcix. Fig. 123, PI. cii, is 
through the cerebellar (?) folds {cer. f.). Fig. 133, PL civ, is just in front of the 
cerebellar folds; the thin roof of the fourth ventricle here would seem to correspond 
[to the valve of Vieussens. Fig. 134, PI. civ, is through the mid-brain and shows 
the iter and developing optic lobes. The infundibulum {Inf.) in its course backwards 
is met with in this section. Fig. 124 is a little in front of the last section, and from a 
, slightly younger stage. The part of the brain here met with is the third ventricle, as 
i Bull. U. S. F. C. 89 16 
