Houser, Zhe Neurones of a Selachian. 135 
marked differences to be observed between the tigroid-bodies 
of the trochlear and the oculomotor neurones, respectively. 
The contrast in both the form and the arrangement of the 
tigroids is quite evident. The distinction is a real one, not due 
to variation in the action of reagents, for I have several instances 
showing the marked contrast in one and the same section. A 
structural difference here is a noteworthy fact, since the two 
neurones are both of the somatic motor type, entirely equiva- 
lent morphologically. 
3. The Ependyma. 
The general character of the ependyma of the midbrain is — 
represented in Fig. 21, ef., while Fig. 61 illustrates the details 
of cell-organization. The outline of a representative cell is 
somewhat lance-like, the length four times the breadth, the 
pointed extremity touching the ventricle and the greatest 
breath at a point further removed. The interior of the cell is 
occupied almost entirely by the nucleus. It is really difficult 
to detect any cytoplasm at all except ina small area at the 
base of the ependymal fibre. The observer has the impression 
forced upon him that most of the cytoplasm during the course 
of growth has passed over into the cell-process. The structure 
of the nucleus is reticulated to an almost extreme degree. 
Some strands of the reticulum are relatively coarse, but many 
of them are so tenuous as to lie almost beyond the capacity of 
the microscope. 
The ependymal fibre takes a course which is almost straight 
during the first part of its length, but toward the outer limit 
of the central gray matter crooked turns occur, and branches 
are given off. The diameter of the process remains uniform 
throughout except for slight swellings which occur at intervals. 
The fibre terminates at the periphery of the brain. 
The fact that the ependymal fibre becomes irregular and 
branches only after it leaves the central gray matter for the 
newer additions outside may have a phylogenetic significance, 
indicating that at one time the process had no farther course 
than the outer limit of this most ancient nerve-substance. But 
