. 261 
From the common stem of the „thalamic‘“ and olfactory roots, a 
line of ganglion cells passes to the walls of the anterior head cavity 
{fig. 11 a) with which they unite in the same manner as ganglion 
cells from the trigeminal and trochlear Anlagen unite with the 
walls of the mandibular cavity. We must therefore believe the rudi- 
mentary muscle formed from the walls of this cavity to have been 
originally innervated by one of the two nerves united in the common 
stem, i. e. by the primitive „thalamicus“ or by the primitive olfactorius. 
While it seems impossible to ascribe the cavity with absolute cer- 
tainty to one rather than to the other of these nerves, there is yet a 
high degree of probability that we have here to do with the los} 
motor elements of the olfactory segment. I will review the facts from 
which this conclusion is reached. There are two distinct dorsal nerve 
roots, connected with two distinct ganglionic enlargements, ciliary and 
olfactory, each fusing with the ectoderm and each acquiring a secon- 
dary attachment to the brain by means of a permanent nerve, oculo- 
motorius in the one case, olfactorius in the other. In my former paper 
(loc. cit.), I have shown that the oculomotorius is primarily an out- 
growth from the ciliary ganglion, formed by the migration and prolon- 
gation of ganglion cells, that it is undoubtedly originally sensory, since 
at the time of its formation the muscle cells which are later supplied 
by the oculomotorius have not yet been formed in the walls of the 
praemandibular cavity, while a thickening of the ectoderm, without 
doubt sensory, is directly connected with the ciliary ganglion, and 
thence by means of the oculomotorius with the brain. We have there- 
fore in neural outgrowth, ganglia, sensory epithelium and secondary 
nerve attachments, the primary’ sensory elements of two head segments, 
an anterior and a posterior one; but we have more, there are also 
two head cavities, so widely separated in their origin by the Anlage 
of the infundibulum, that they cannot well be ascribed to the same 
head segment. The walls of each cavity develop muscle cells, and 
to the walls of each cavity pass neural cells from the common An- 
lage of the ,,thalamic‘ and olfactory nerves. We have consequently 
the primitive motor elements of two head segments, an anterior and 
a posterior one. 
With the disappearance of the ectodermic thickening connected 
with the ciliary ganglion, and with the high development of the eye 
muscles formed from the walls of the praemandibular cavity, the 
sensory elements of the posterior of these two segments are sacrificed 
to the motor. With the atrophy of the muscle formed from the walls 
of the anterior segment, and with the high specialization of the nasal 
