236 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
lie above the superior oblique muscle. Thus the permanent trochlearis 
arises from two sources, from the brain and from ganglion cells.” Fi- 
nally, Kupffer (’91) stated that he had found a nerve in Ammocostes, 
which he thought to be the trochlearis (for reasons not clear to me), 
directly connected with the second epibranchial ganglion. Were this 
opinion correct, the trochlearis would be the serial homologue of a 
branchial (dorsal), not of a spinal dorsal nerve. 
From this summary of previous embryological evidence bearing on 
the question of the morphology of the trochlear nerve, it is clear that 
little support is given to the view, based on the later histological struc- 
ture and relations, that it is morphologically a ventral segmental nerve. 
Only Kastschenko (88) finds the nerve in early stages fibrillar in 
structure. The following evidence, however, leads me to conclude that 
its mode of development is the same as that of the oculomotorius and 
abducens, and that therefore it must be regarded, like these, as a ven- 
tral (medullary) nerve. I first find the trochlearis in sections of embryos 
of 19-20 mm. as a fibrillar nerve bundle extending from the dorsal con- 
striction between encephalomeres II and III. Two roots are already 
present at this stage, but neither in these, nor in the nerve bundle as 
far as its fibres may be traced in the mesenchymatous tissue at the sides 
of the brain, are nuclei to be found.” While proximally the nerve fibres 
are united in a compact bundle, they distally separate so as to form a 
loose brush of structureless fibres, which are lost in the mesenchyma at 
a considerable distance dorsal to the muse. obliquus superior (Figure K). 
While I am able to offer no direct evidence in favor of the view that the 
fibres of the trochlearis, as above described, are processes from neuro- 
blast cells in the ventral horn of encephalomere III, I hold that they 
are such, since their well known later histological relations support this 
conclusion. Dorso-ventral fibres in this region of the neural tube may 
indeed be traced in embryos of this stage, but their connection with the 
fibres of the trochlearis is not clear to me. The dorsal chiasma of fibres 
is present. Of a ganglion, or of any grouping of cells which might 
is that portion of the trigeminus Anlage which I have for convenience called its 
trochlear portion, which persists for some time in the constriction between midbrain 
and hindbrain vesicles. Since the proof of its morphological value has not been 
given, and since the “ permanent” trochlearis is not developed from the “ primary ” 
trochlearis, as Miss Platt herself states (p. 96), the use of the latter term appears 
to me apt to mislead. 
1 The explanation for this dorsal chiasma may be sought in some physiological 
advantage in coördination gained, but it may also be seen that in case the dorsal 
exit of fibres were of physiological advantage, it would be easy for the fibres to 
