258 THE ORIGIN OF VERTEBRATES 
to a single segment, but are the somatic motor roots belonging to 
the same segments as those to which the trigeminal supplies the 
splanchnic roots ? 
4. Do the mesoderm segments, which give origin to the eye- 
muscles, and therefore do the head-cavities of this region, correspond 
with the ‘trigeminal segments? Considering the concentration of 
parts in this region and the difficulty already presented by the want 
of numerical agreement between the prosomatic appendages and the 
prosomatic ccelomic cavities in Limulus, it may very probably be 
difficult to determine the actual number of the mesoderm segments. 
5. Is there anatomical evidence that the ganglion of origin of the 
motor part of the trigeminal nerve is not a single ganglion, but a 
representative of many, probably seven ? 
6. Is there anatomical evidence that the ganglia of origin of the 
oculomotor and trochlear nerves represent many ganglia ? 
7. Is there any evidence that the organs originally supplied by 
the motor part of the trigeminal nerve are directly comparable with 
prosomatic appendages ? 
It is agreed on all sides that in this region of the head there is 
distinct evidence of double segmentation, the dorsal mesoderm segments 
giving origin to the eye-muscles, and the ventral segments to the 
musculature innervated by the trigeminal nerve. Originally, accord- 
ing to the scheme of van Wijhe, two segments only were recognized, 
the dorsal parts of which were innervated by the IIIrd and IVth 
nerves respectively. Since his paper, the tendency has been to 
increase the number of segments in this region, as is seen in the 
following sketch, taken from Rabl, of the history of cranial 
segmentation. 2 
HisToRY oF CRANIAL SEGMENTATION. 
The first attempt to deal with this question was made by Goethe 
and Oken. They considered that the cranial skeleton was composed 
of a series of vertebra, but as early as 1842 Vogt pointed out that 
only the occipital segments could be reduced to vertebra. In 1869, 
Huxley showed that vertebra were insufficient to explain the 
cranial segmentation, and that the nerves must be specially con- 
sidered. The olfactory and optic nerves he regarded as parts of 
the brain, not true segmental nerves; the rest of the cranial nerves 
