THE NERVOUS SYSTEM 263 
The motor nuclei of the oculo-motor, trochlearis, abducens, 
and hypoglossus nerves lie in the same plane as the motor nuclei 
of the spinal nerves, 7.e., in the line of prolongation of the ventral 
horns of the gray matter. The motor nuclei of the trigeminus, 
facialis, glossopharyngeus, vagus, and spinal accessory on the 
other hand lie at a more dorsal level, and the roots emerge there- 
fore above the level of origin of the others. It will be noted that 
these are the nerves of the visceral arches, whereas those cranial 
nerves that continue the series of spinal ventral roots innervate 
myotomic muscles, like the latter. Similarly the ganglia of the 
pharyngeal nerves (V, VII, LX, and X) differ from spinal gangliz 
in certain important respects: the latter are derived entirely 
from the neural crest, whereas a certain portion of each of the 
primary cranial ganglia is derived from the lateral ectoderm of 
the head, as noted in the preceding chapter. Thus the pharyn- 
geal nerves form embryologically a class by themselves, both 
as regards the medullary and also the ganglionic components. 
1. The Oljactory Nerve. The embryonic origin of the olfactory 
nerve has been a subject of much difference of opinion: thus it 
has been maintained by a considerable number of workers that 
it arises froma group of cells on each side situated between the 
fore-brain and olfactory pits; some of these maintained that 
these cells arose as an outgrowth from the fore-brain, others 
that they came from the epithelium of the olfactory pit, and 
yet others that this group of cells, or olfactory ganglion, was 
derived from both sources. This group of cells was supposed 
by some to include a large number of bipolar neuroblasts, one 
process of which grew towards the olfactory epithelium and 
the other towards the fore-brain, entering the olfactory lobe 
and ending there in terminal arborization. This view is, however, 
in conflict with the ascertained fact that the fibers of the fully 
formed olfactory nerve are centripetal processes of olfactory 
sensory cells situated in the olfactory epithelium. 
The most satisfactory account of the origin of the olfactory 
nerve in the chick is that of Disse. This author finds two kinds 
of cells in the olfactory epithelium of a three-day chick, viz., 
epithelial cells, and germinal cells which become embryonic 
nerve-cells or neuroblasts. At this time the olfactory epithelium 
is separated from the wall of the fore-brain by only a very thin 
layer of mesenchyme. Early on the fourth day axones arise 
