EMBRYOLOGY OF THE SFA BASS. 
243 
The coiitiuiious dorso-ventral fin of the larva is indicated at a comi)aratively early 
stage in embryonic life by a groove in the nervous layer of the ectoderm (/. gr., Figs. 
88, PI. xcviii ; 99, PL xoix). The groove first appears j nst before the tail begins to fold 
off, at the posterior end of tlie body and on the dorsal side (Fig. 88, /. gr.). As the 
tail is folded off the groove extends ronud its tip to the ventral side (Figs. 99, 100, 
PI. xoix), and as the folding off of the tail continues, the formation of the groove 
progresses in an anterior direction on both dorsal and ventral surfaces. In the Bass, 
1 as in other Teleosts, after the tail has grown out a short distance it becomes laterally 
compressed, the dorso-ventral fin fold (d. /., v. /., Figs. HI, 112, PI. ci; 119, PI. on) 
developing at the same time. By this means the tail end of the embryo is made to lie 
against the yolk on its side. From the start the cells which line the fin groove (/. i/r.. 
Figs. 88, 99), and wliich belong to the second stratum of the nervous layer, are larger 
and not so flattened as the surrounding cells. When the fin fold begins to develop 
(Fig. 112) the groove extends into it, the cells of the second nervous stratum retaining 
an approximately cubical shape. The cavity of the fin fold (Fig. 112) disappears before 
the close of embryonic life, but the double sheet of nonvaeuolated cells persists. 
Eye. — The development of the eye does not differ from the generally accepted 
; account for the Teleosts, except in regard to the lens, and it may therefore be run over 
very briefly. The solid optic sacs (Fig. 64, PI. xcvi) are separated from the brain by 
a fissure which extends from above downwards and, as in other Teleosts, from behind 
forwards, and hence the position of the optic nerves in later stages (Figs. 135, PI. 
CIV, and 149, PI. CVii). The cavity of the optic sac is established in the way already 
described for the neural cord, and the wall of the sac is at first made up of a single 
layer of cells (compare Fig. 72, PI. xcvii, through the still solid optic sac, with Pig. 80, 
PI. XCVII, after the cavity is formed). In the folding of the optic sac to form the optic 
cup there is nothing which calls for special attention. The inner layer of the cup 
thickens and forms the retina, the outer layer becomes transformed into the ordinary 
stratum of flat cells (pigmented epithelium of the choroid. Figs. 80, PI. xcvii, 97, PI. 
xcix, 108, PI. 0 ). The choroidal fissure is still present at the time of hatching (Fig. 
151, Pi. evil). 
The development of the lens in the Bass affords an interesting illustration of how 
one modification in the embryo leads to another. In the trout the head region of the 
embryo is early lifted up above the yolk, so that it is possible for the lens to develop, 
I as it does in the ordinary position. It is developed as a solid thickening in which the 
epidermic stratum takes no part (Heuiieguy). In the Bass, however, the whole em- 
bryo, head as well as trunk, is buried in the yolk (Fig. 97, PI. xcix), and it is only 
late in embryonic life, long after the lens has formed, that the head begins to be 
folded off (Figs. 130, PI. cm, and 135, PI. civ). On glancing at Figs. 72, PL xcvii, 80, 
PI. XCVII, and 97, PL xcix, it is seen that it is impossible, because of the reason just 
mentioned, for the lens to develop on the side of the head, as in other vertebrates — the 
layer here consisting of periblast {p) and not ectoderm. 
The lens makes its first appearance while the optic sacs are still solid, as a thick- 
ening of the nervous layer of the ectoderm. Its position is on the dorsal surface in the 
angle between the ectoderm and periblast (/«,. Fig. 72, PL xcvii). The thickening 
becomes transformed into an open invagination which grows down between the optic 
cup and the periblast (bt., Fig. 80, PL xcvii). The inner wall of the invagination is 
made up of columnar cells (c. ep.), and is therefore conspicuous, but the outer wall is 
