EMBRYOLOGY OF THE SEA BASS. 
245 
' the two sacs persists the connecting portion of the sensory furrow, and behind the 
i auditory sacs the furrow is continued for some distance. The posterior portion {1. 1.) 
i of the furrow constitutes the anlage of the lateral line. 
In the next stage (Fig. 148, PI. cvi) the sensory furrow has definitely separated 
! into its three derivatives. The auditory sac has closed, and with this closure was 
naturally brought about the division of the furrow. The three derivatives at this 
ri stage have not yet begun to move away from each other. In a subsequent stage 
I (Fig. 149, PI. evil) they are, however, wide apart. In this stage the gill slit {g. s.) has 
|j broken through, and just in front of it is the branchial sense organ. The auditory 
I sac is already overgrown (see dorsal view) by the medulla, and the anlage of the lat- 
eral line {1. 1.) has moved some distance backwards fi’om its original position in front 
;i of the somites. In a still later stage (Fig. 150) the lateral line anlage has grown still 
I farther back, and is incomple‘;ely divided into three “sense organs of the lateral line.” 
i After this brief survey we may return for a more detailed description of these 
i[ parts. The series of transverse sections (Figs. 68-71, Pis. xcvi and xcvu, numbered 
!i from behind forwards) is from a stage slightly less advanced than Fig. 147, PI. cvi. 
^ The anterior section (Fig. 71) is through that portion of the furrow which has already 
begun to difierentiate itself into the branchial sense organ {b. s. o.). The greater 
thickness of the proximal wall of the groove, and the cavity of invagination need no 
description. In going backwards from this section an unbroken though comparatively 
shallow furrow leads ^to the auditory sac (Fig. 79). Here the invagination has a 
greater width and depth than at any other spot. In passing towards the posterior 
limit of the auditory sac the invagination draws away from the brain (Figs. G8 and 69), 
as is indicated in the surface view. Fig. 147. In Fig. 70, through the middle of the 
ear sac, where the invagination is nearest the brain, it is impossible to speak of an 
auditory nerve, for here there is direct continuity between the brain cells and the lining 
cells of the invagination. The auditory nerve becomes recognizable as a prolifera- 
tion of cells only where the sac is separated from the brain by some of the surface 
ectoderm (Fig. 69, a. n.). Behind Fig. 68 the invagination becomes shallower and 
narrower, constituting the anlage of the lateral line. 
Figs. 78 and 79, PI. xcvii, are from a stage intermediate between Figs. 147 and 148, 
but in which the auditory sac had closed. Fig. 79 is through the branchial sense 
organ {b. s. o.), which is deeper and of a more compact shape than in early stages. 
Fig. 78 is through the posterior part of the closed auditory sac {a. s.) and the anterior 
part of the lateral line (1. 1.). As the section indicates, though the sensory furrow has 
broken up into its several parts, these parts still closely adjoin one another. 
Now that the history of the sensory furrow has been carried to this point, it will 
be more convenient to treat each derivative separately. 
Ear . — The ear, after its constriction from the surface and the rest of the sensory 
furrow, forms a closed sac, the wall of which is made up of columnar cells (Fig. 95, 
PI. xcix). One side of the sac applies itself pretty closely to the medulla, and during 
later stages the cells of this wall become greatly flattened. Two stages are given in 
Figs. 114, PI. ci, 130, PL cm, the latter showing the condition of the ear at the 
time of hatching. At this time the columnar sensory elements are restricted to a 
rather closely circumscribed area. The overarching of the ear by the medulla has 
already been mentioned. 
