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 1 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 Illrd 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 Eabl, of the history of cranial 

 segmentation. 



HlSTOEY 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 vertebrae, but as early as 1842 Vogt pointed out that 

 only the occipital segments could be reduced to vertebrae. In 1869, 

 Huxley showed that vertebrae 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 



