240 WM. A. KEPNER AND J. R. CASH 
in the fundus of the mature pit there are no rhabdites, it is sug- 
gested that, as the cells of the developing epithelium are physio- 
logically differentiated, they lose their power to elaborate rhab- 
dites and develop a greater capacity to secrete a permanent, 
refractive, mucus-like glea which protects the greatly exposed 
and extremely sensitive fundus of the pit. So we draw the con- 
clusion that the only difference between the mucus secreted by 
the cells of the general epithelium and the mucus which com- 
poses the ‘homogeneous mass’ of the ciliated pit is that the 
‘homogeneous mass’ is permanent, perhaps more dense, and 
withstands the action of reagents better than does the temporary 
secretion of the general epithelial cells. We have made compari- 
son of these two substances by staining with methylin blue, in 
which case they stain alike, both staining in living specimens a 
rather dark blue as contrasted with the intensely dark blue of 
the rhabdites. So much for the development of the epithelium 
of the ciliated pit and its secretion product. 
DEVELOPMENT OF THE GANGLION OF THE CILIATED PIT 
As has been stated previously in this paper, beneath the form- 
ing ciliated pit there is a mass of cells which we take to be mesen- 
chymal in origin. This statement is made in abeyance since we 
are not concerned at present with the origin of the ‘brain.’ We 
have, however, been able to see that the ‘brain’ arises from these 
cells. But the important point which we endeavor to make is 
that the ciliated pit-ganglion has a distinct origin from the 
epidermis. 
With the earliest formation of the pit-depression at the sur- 
face of the body there occur mitoses in its epithelium which 
send into the mesenchymal region a number of cells which locate 
themselves between the epithelium of the fundus and the mesen- 
chymal cells which form the ‘brain,’ as shown in figure 2. There 
is, however, a distinction at the very outset between the cells 
of the ‘brain’ and those which are forming the ciliated pit-gan- 
glion, as shown by the above figure. This proliferation of cells 
arising from the epidermis continues to grow with the develop- 
ment of the superficial part of the pit. 
