230 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
olfactory pit and brain wall as these separate from each other. I regret 
that I am not yet in position to describe the later differentiation of 
these cells, but it appears to me not improbable that they enter into 
the Anlage of the olfactorius. The evidence given by many histologists, 
from Schulze to Retzius, establishes the ganglionic character of the 
olfactorius; while the evidence presented by His (89°) appears confirma- 
tory of the view of Beard, that the olfactory plate is to be regarded as 
the anterior of the sense organs of the lateral line, since from it are 
derived, at least in part, the ganglionic cells of the olfactorius. There- 
fore, if neural-crest cells also entered into the Anlage of this nerve, it 
must be regarded as homodynamous with the sensor component of a 
dorsal segmental nerve. 
2. ABDUCENS, 
Another of the eye-muscle nerves, viz. the abducens, is now (65 so- 
mites, 10 mm.) differentiated. (Compare Figs. 20 and 21 with Fig. 19.) 
The latter nerve has arisen as an outgrowth from neuroblast cells in the 
ventral horn of encephalomere VII, and its roots retain connection with 
this encephalomere until the latter disappears. Zimmermann (91) stated 
incorrectly that its connection in Squalus is with the neural segment 
which corresponds with my encephalomere VI. Dohrn (90°) describes 
the nerve as having its origin from the neural tube opposite the otic 
capsule, and between nerves VII and IX. Its position in different 
Vertebrates seems inconstant, Some investigators (Orr, ’87, Waters, ’92, 
and Herrick, ’92) have stated that in the forms studied by them it arises 
from the hindbrain neuromere corresponding with encephalomere IV of 
my figures, In the chick and swine I have found that its roots are 
in connection with encephalomere VI, whereas in Necturus its fibres 
may be traced from the muse. rectus posterior to a point behind the ear, 
and thus have, as I believe, their origin from a segment of the hindbrain 
corresponding with encephalomere VII. At least, in this form, as in 
Squalus, it appears as a post-otic nerve. Dohrn (91) gave a careful 
account of the early stages of its development in embryos of various 
Selachii. He states that the nerve first appears at a stage corresponding 
with Balfour’s stage L, arising by two roots which unite at a short dis- 
tance from their point of exit from the ventral wall of the neural tube. 
In Mustelus the roots are more numerous than in the other forms exam- 
ined, there being as many as six on each side of the brain. The roots 
are directed backward, as in the case of spinal nerves, but later form a 
network from which arises the stem of the nerve; this runs forward, 
