336 End-Organs of the Trigeminus and Lateralis Nerves 
In chromic acid material the canal has sometimes the appearance of a 
pinching in of the epithelium, as in Fig. 9, and at others the appearance 
of a well-formed groove as in Fig. 10; in formalin sections it is ordinarily 
deeper and narrower, as in Fig. 7. The central ridge of spindle cells has 
the same relation to the canal in chromic acid sections as is shown in 
those preserved in formalin, but in Fig. 9, where the sides of the canal 
have been pinched together, this relation is not so easily recognizable. 
In some sections the ridge runs only as far as the lumen (Fig. 10), 
and in others, as in Fig. 9, it crosses the lumen to dovetail into long 
slender cells of the same kind resting on the basement membrane, thus 
bisecting the tube. ‘This bundle of spindle cells has, in cross-section, the 
appearance of a plug, and the line of cleavage down the center is 
occasionally well marked. 
There is still a third typical cross-section of the tube (Fig. 10). In 
it the spindle-celled ridge has apparently been torn in two across the 
middle along a plane parallel with the surface, leaving a decided ridge 
rising from the floor of the tube. In this lower ridge are found several 
single sensory cells (Fig. 10, s). 
From the preceding statements two facts stand out prominently. The 
first is the embryonic condition of the canals as compared with the typical 
canals of higher vertebrates. In Bdellostoma the canals remain through- 
out life entirely within the epidermis. At no stage of growth do they 
acquire a dermal sheath. They thus represent in their adult condition a 
stage of growth present only in the embryos of higher vertebrates. ‘The 
second is the relatively low stage of differentiation of the sense organs. 
The dermis is depressed underneath the canals, but the edges of the 
erooves thus formed do not grow upward to enclose the epithelial canal. 
The dermal groove is never deep enough to enclose the sense organ. 
CINCINNATI, O., April, 1907. 
