BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
258 
paper for cranial ventral nerves in Selachii, leads to the same conclusion. 
The visceral cleft which defined anteriorly the splanchnic part of the 
fourth segment is not ontogenetically evident in Squalus. Kupffer (93) 
has possibly seen evidence of a rudimentary cleft between the mouth and 
the hyomandibular cleft of Acipenser. And possibly this cleft may be 
represented in the “ Pseudobranchialrinne ” of Amphioxus. 
e. RELATIONS or ENCEPHALOMERE III. 
As in the case of the four posterior hindbrain segments, the study of 
the development of the nerves connected with encephalomere III (Hinter- 
hirn) gives the clue to the primitive relations of this primary vesicle. 
The neural-crest cells proliferated from it pass ventrally into the man- 
dibular arch. From a part of these a large ganglion is formed (the 
Gasserian), through which pass the motor fibres, whose nucleus is, at 
least in part, in encephalomere III, to innervate the musculature of the 
first visceral (mandibular) arch. We have thus the splanchnic elements 
ofa cranial segment. In the Table of Nerve Relations (p. 253) the troch- 
learis has been given as the ventral (somatic) nerve of this segment. 
The evidence in favor of this view has already been stated, and consists 
in the facts that it innervates musculature derived from dorsal (somatic) 
mesoderm, that its fibres develop as processes of neuroblasts in the neural 
tube, and that its histological relations and structure in the adult show 
it to be a purely motor nerve with motor nucleus in the ventral horn of 
encephalomere ITI. I regard the mouth as representing the fused visce- 
ral clefts which bounded anteriorly the splanchnic portion of this seg- 
ment. We have thus all the essential elements of a head metamere. 
Jf. RELATIONS or ENOEPHALOMERE II. 
From the simple dorsal expansion of encephalomere II are proliferated 
cells which pass ventrally and fuse with the skin to form the meso- 
cephalic ganglion * lateral to the Ist somite (Figs. 17 to 20). Although 
this ganglion never becomes connected with the midbrain (encephalo- 
mere II), since its fibres enter the brain through the r. ophthalmicus 
profundus V, it must in my opinion be regarded as a segmental gan- 
glion comparable with those of the following cranial nerves ; the oph- 
thalmicus profundus must likewise be considered as a dorsal nerve 
homodynamic with the succeeding cranial nerves. Its want of motor 
fibres may be explained as resulting from phylogenetic loss, since in 
1 This ganglion is homologous with the first trigeminus ganglion of Cyclostomes, 
