NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 277 
(encephalomeres) may be considered as the primitive nervous centres of 
nerves which corresponded numerically with visceral arches. If they 
were such, then one of the encephalomeres (IV) affords evidence of a 
lost visceral arch. 
Although the structure of myelomeres and encephalomeres is seen to 
be different, yet in the stages of embryonic development, where both 
are present, the latter are seen to have segmental value from the fact 
that corresponding with them there is an equal number of somites. 
These somites, as exemplified in the 3d (van Wijhe’s), are morphologically 
comparable and serially homologous with trunk somites. I conclude, 
then, that there was a primitive correspondence between neuromerism, 
mesomerism, and branchiomerism. 
The development, histological structure, and relationships of the 
eye-muscle nerves (II, IV, and VI) show them to be the serial homo- 
logues of ventral spinal nerves. Like the latter (His), they develop 
as axis-cylinder processes of neuroblasts in the ventral horn of the 
neural tube. 
Pre-otic and post-otic metameres, like their integral parts, are serially, 
homologous with one another. Therefore, if the latter are serially 
homologous with trunk metameres the former must be also. Table III. 
(p. 253) summarizes my opinion as to the primitive composition of 
metameres I to VII. I regard the r. opthalmicus profundus as a seg- 
mental dorsal nerve belonging to metamere IT, while the oculomotorius 
is its ventral root. The trochlearis is the ventral nerve of metamere 
III, and the abducens represents the ventral nerves of metameres IV 
to VII. 
There are five mesomeres alternating with six neuromeres in the otic 
and pre-otic regions of the Vertebrate head. Probably eleven neuro- 
meres are finally included in the head of Squalus. The evidence of the 
numerical correspondence of neuromeres and mesomeres shows that 
there is no more reason for believing that somites have been lost 
anterior to Platt’s (anterior) somite, than that neuromeres have been 
lost anterior to the primary forebrain. 
In agreement with van Wijhe, I homologize the mouth of Amphi- 
oxus with the left half of the mouth of Craniota. The first pair of per- 
manent visceral clefts in Amphioxus are exactly homologous with the 
hyomandibular clefts of higher Vertebrates. The eight visceral clefts 
possessed by Amphioxus at its “ critical stage” (Willey) are exactly 
homologous with the eight morphological clefts found in some Selachii 
and Cyclostomes. 
