660 
Position of derivatives with reference to gill slits. 
On the twenty-fifth day there are five gill slits. Of these the middle 
one, the third, is the first to appear, breaking through on the fifteenth 
day. Later, on eighteenth day, the fourth cleft makes its appearances. 
The second cleft opens on the twenty-first day. The first and fifth 
come in at about one time, the twenty-fifth day. It is the third, or 
middle cleft, that corresponds to the single slit of the embryo Serranus. 
It opens directly beneath the auditory sac (Fig. 2). In front of it lies 
the pre-auditory thickening, occupying thus about the same position 
as does the single branchial sense organ of the bass. Behind it at 
some little distance lies the post-auditory thickening. 
LAA GOs AS 
Fig. 2. Anterior half 
of salmon embryo 25 days. 
Extent and position of ecto- 
dermal thickenings plotted 
from sections. 4. s. Auditory 
sac. Pr.a.th. Pre auditory 
thickening. Lat. l.anl, La- 
teral line rod, passing an- 
teriorly into wider and less 
well defined tract, the two 
constituting the post-auditory 
thickening. 
Lat. l. ani. 
When the first and second clefts make their appearance, the pre- 
auditory thickening lies above them, and it bifurcates anteriorly (Fig. 2). 
The position of the bifurcating limbs is about the same as that of the 
infra- and supra-orbital rods (canals) traced by MirropHanow. In 
the bass, Wırsox found that the rod of cells running forwards from 
the branchial sense organ, also bifurcated anteriorly. 
The post-auditory thickening, lying back of the third slit, becomes 
separated from it, during the period from the fifteenth to twenty-fifth 
day, by a lengthening area of comparatively thin ectoderm. In this 
area the fourth and fifth slits make their appearance. Behind the 
last slit remains a wide and not very clearly defined thickened tract, 
which quickly narrows posteriorly passing into the rod-like thickening 
of the lateral line. The lateral line rod on the twenty-fifth day is a 
continuous rod, rather sharply outlined, but not yet constricted off from 
the nervous layer of the ectoderm. It is still a mere thickening of 
this layer. 
University of North Carolina 
Chapel Hill N.C. U.S. A. 
