212 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
truthfully said that cranial nerves differ from spinal in that the former 
extend laterad and the latter mediad of the mesomeres. We must con- 
clude that dorsal nerves were in all probability, as in Amphioxus, re- 
lated to the septa between myotomes. Finally, the distinction made by 
His, in the case of dorsal cranial nerves, between dorsal (sensor) and 
lateral (motor) roots, has, with the knowledge of the facts above stated, 
an anatomical and physiological rather than a morphological interest. 
I therefore sce no escape from the conclusion that the occipital region of 
the head is not a region sui generis, and I pass to the consideration of 
the pre-occipital segments. 
To those who are deeply impressed with the differences between post- 
otic and pre-otic regions of the Vertebrate head, it is necessary to em- 
phasize the following fundamental resemblances in the segments of these 
two regions. (1) Pre-otic and post-otic encephalomeres have been 
shown to be morphologically comparable. (2) The dorsal nerves con- 
nected with these, and (3) the visceral arches which these nerves supply 
are in these two regions serially homologous. Moreover, as evidence 
pointing in the same direction, it may be stated that (4) a post-otic 
nerve innervates pre-otic. musculature. Furthermore, the serial homol- 
ogy of pre-otic and post-otic somites appears established by the fact 
that (5) a pre-otic somite (van Wijhe’s 3d somite) is a segment of the 
dorsal mesoderm. That it is such seems clear, for it is defined 
anteriorly and posteriorly by well marked constrictions (observed by 
several investigators), it becomes differentiated into myotome and sclero- 
tome, and its musculature appears first in its median wall, and becomes 
innervated by a ventral nerve (abducens) serially homologous with ven- 
The fact that the primitively dorsal mesoderm of the 
y to form the splanchnic musculature, as 
lachii, and Amphibia, is not a basis 
tral spinal nerves. 
pre-otic region grows ventrall 
has been stated for Cyclostomes, Se 
for a fundamental distinction between post-otic and pre-otic regions, 
since this is the method of formation of splanchnic mesoderm through- 
out the length of the body in Amphioxus. In this respect, as in 
respect to the nerves, the head shows more primitive conditions than 
‚ade and a half shows 
the trunk. Since the literature of the last dec 
little agreement of opinion as to the morphology of the eye-mu 
oculomotorius and the trochlearis, and 
given which tends to 
r briefly the 
The 
scle 
nerves, more especially the 
since in the preceding pages evidence has been 
reconcile existing differences, it is important to conside 
bearing of their morphology upon that of the pre-otic segments. 
more recent attempts to classify the eye-muscle nerves as dorsal, 
