EMBRYOLOGY OF THE SEA BASS. 
233 
Subsequently this stratum is perforated and a true opening is established. After the 
opening is once established there is formed round it a shallow depression of the gen- 
eral surface, so that in transverse sections the opening comes to be funuel-shaiied {g. 
s., Fig. 114, PI. ci). The external appearance of the gill opening may be gathered 
from Figs. 149 and 150, PI. cvii, g. s. As I have said, tliere is but this one opening into 
the branchial chamber during embryonic life. The remaining gill slits appear a day or 
two after hatching in front of the embryonic slit, but I have not studied their mode of 
formation. 
Postanal gut . — In the stage which has just been under examination the tail had 
barely begun to develop. A few hours later (surface view from below, Fig. 98, PI. 
xcix), it becomes a prominent feature, and in the course of its development comes to 
contain the greater part of the postanal gut {p. a. g.). lu this stage (Fig. 98) the cavity 
of KuptfePs vesicle has entirely disappeared. Indeed the cavity disappears' in a much 
earlier stage of the development of the tail, as will be seen in Fig. 89, PI. xcvin, which 
is from a stage but slightly more advanced than Figs. 88 and 84, PL xcviii. The oblit- 
eration of the cavity is brought about by the proliferation of the cells of its own wall. 
After the disappearance of its cavity KupflfePs vesicle is not distinguishable from the 
rest of the postanal gut, which at its posterior end gradually increases in size (Fig. 98) 
before vanishing in the caudal mass. The transverse sections. Figs. 99 and 100, PI. 
xcix, and Fig. 102, PI. c, are all from an embryo nearly like Fig. 98. The two former 
sections show the condition of the gut within the tail, while Fig. 102 shows the condi- 
tion of the tract p. a. g., which the folding off of the tail has not yet reached. Eeferring 
them to Fig. 98, Fig. 99 liesiu the Plane 1, Fig. 100 in Plane 2, and Fig, 102 in Plane 
3, The position of the anus (not yet broken through) in the surface view is at a. Fig. 
99 occupies the same relative position that Fig. 90 had in an earlier stage, is just in 
front of the neurenteric streak, and the eutodermic mass, p. a. g., is composed of the 
fused liostanal gut and notochord. In Fig. 100 the postanal gut and notochord are 
separate. In front of the tail. Fig. 102, the postanal gut, a. g., is less massive than 
farther back. 
The postanal gut reaches the height of its development in the stage shown in 
Fig. 98. Almost immediately atrophy sets in. The atrophy begins at the anterior 
limit of the tail, and travels backwards and forwards. The sections, Figs. 99, 100, 
101, and 102, are from an embryo in which the atrophy has just begun, and Fig. 101 is 
through the place where it starts. As will be seen in this figure, the atrophy of the 
gut leaves certain cells to mark its former position. These cells will again be touched 
on in dealing with the subnotochordal rod, the caudal extension of which they repre- 
sent. The same cells in a later stage and farther back in the tail are again shown iu 
Fig. 109, PI. c, and Fig. Ill, PI. ci, s. n. r. The last part of the postanal gut to 
disappear is its swollen end. 
Subsequent history of the alimentary canal . — The formation of the anus takes place 
before hatching, and there seems to be no ectodermal invagination to form a procto- 
dmum. The mouth breaks through a couple of days after hatching. Shortly after 
the mouth appears, the cells which line the alimentary canal lose their embryonic 
appearance and come to look much like au adult mucous membrane ; they secrete a 
cuticle and their limiting surface is no longer smooth (Fig. 144, PI. civ, al. c.). In 
the just hatched larva, the condition of the alimentary canal in that part of the trunk 
which has been folded off from the yolk is shown iu Fig. 126, PI. on. 
