98 The American Naturalist. [February, 
gion arise almost entirely in portions where in the protovertebra 
connective tissue originates, and vice versa. There is no differ- 
entiation of myotomes and sclerotomes in the proximal somites. 
There are two primary nerves in the cephalic portion, the V. and 
VII—VIII., but these do not arise from a continuous neural ridge, 
The cephalic border of the neural ridge forms a delicate strand 
uniting with the triangular part of the trigeminus Anlage, which 
becomes the ciliary ganglion. In later stages, answering to the 
direction of this delicate strand, extend the oculomotor and troch- 
lear nerves. The oculomotor and trochlear nerves are thus to be 
considered as secondarily derived from the trigeminus, and the 
eye-muscles perhaps from the musculature of the first branchial 
arch innervated by the V. From researches on Selachians, birds, 
and mammals it is concluded that the III. and IV. nerves arise on 
the dorsal border of the midbrain. The primary nerves of the 
caudal region of the head are the IX., X., and hypoglossus, the 
latter consisting of the ventral roots of the region. The IX. ani 
X. arise from a continuous neural ridge in a series with the dorsal 
roots of the true spinal nerves. The opinion of Beard, that the 
“ Anlagen ” of the dorsal cranial and spinal roots develop prior 
to and independent of the neural tube, is erroneous. The homol- 
ogy of the spinal ganglia to the parapodial ganglia of Annelids 
cannot be established till it is proved that the spinal ganglia grow 
out of the ectoderm independent of the neural tube. Rabi bases — 
his observations on embryos of Torpedo ocellata. The unseg 
mented mesoderm of the head in the Craniota he compares with * 
the unsegmented forward extension of the first primitive segment 
in Amphioxus. In the head region of Amphioxus are two stout | 
nerves, which cannot be compared with spinal nerves. Rabi mee oe 
they may be homologous with the V, and VII.—-VIL of the Craniota. 
In reply to Rabl, Dohrn (90a) notes that the former repeats: 
the mistake of Balfour in deriving the dorsal roots of the spinal 
nerves from the neural ridge. In all Selachians the dorsal roo 
_ of the spinal nerves grow out of the ganglia into the neural tube. 
_ The sensory fibres of the cranial nerves (V., VIL-VIL, 1X. and) 
grow out from the ganglia into the brain, while the motor fibres 
spring from the cells of the lateral columns and enter the ganglia. 
