659 
The Salmon may now be added to the list of forms in which this 
common Anlage has been found. The Anlage in the Salmon divides, 
as in Serranus, into three parts, the middle becoming the auditory 
sac, the posterior the rudiment of the lateral line, and the anterior 
remaining as a very noticeable thickening situated above the anterior 
gill clefts. This thickening remains comparatively undifferentiated at 
any rate as late as the twenty-fifth day. The original Anlage is in 
the Salmon, as in Selachians, a thickening and not an invagination. 
Formation ofthe Anlage. The embryonic shield begins about 
the twelfth day to thicken in the median line to form the neural keel. 
The keel forms rapidly and on the thirteenth day is deep and di- 
stinctly marked off from the surrounding shield. In the formation of 
the keel, cell multiplication doubtless plays a part, as is evidenced 
by the nuclear figures, but cell migration must also occur extensively. 
The shield to begin with is wide and thick (five or six layers of cells), 
but as the keel develops in the middle, its lateral parts thin out greatly, 
the cells presumably shifting their position. 
Now during the formation of the keel, there is left immediately 
on each side of it a comparatively thick streak of ectoderm, fading 
away externally into thin ectoderm. The thick streak extends, at this 
early stage, through the anterior two thirds of the embryo’s length. 
This streak is the lateral sensory Anlage, and it is to be noticed that 
it is not produced as a thickening, but is left over during the 
thinning out of the shield. 
Division of the Anlage. Towards the end of the thirteenth 
day the middle portion of the continuous Anlage begins to be in- 
vaginated. The invagination is the auditory sac, and as it deepens 
during the next day it begins to close up from behind forwards (Fig. 1). 
Fig. 1. Diagrammatic longi- 
tudinal section through common sen- Rat. N S$. Pa. th 
sory anlage, after appearance of audi- 
tory invagination, 4.s. Auditory in- 
vagination. Pr.a.th. Pre auditory 
thickening. P.a.th. Post auditory 
thickening. 
Owing to this method of closure the auditory sac on fourteenth day 
and for some six days later, remains open anterivrly and is continuous 
with the anterior part of the original Anlage, the pre-auditory thicken- 
ing. Posteriorly on the contrary the auditory sac, on the fourteenth 
day and subsequently, is separated from the posterior part of the 
original Anlage by intervening thin ectoderm. The posterior part of 
the original Anlage may be called the post-auditory thickening. 
