3. Is there morphological evidence that the oculomotor and trochlear nerves, which on all sides are regarded as belonging to the trigeminal segments, are not single nerves corresponding each {258}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 coelomic 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, according 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. HISTORY OF CRANIAL SEGMENTATION.