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NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 251 
of encephalomere V (facialis) to the present spiracular cleft, that this 
was once the second visceral cleft instead of the first (disregarding for 
the present the possibility that the mouth represents a pair of gill clefts), 
as it now is. It seems entirely possible that the outpocketing of the 
present first visceral (hyo-mandibular) cleft was originally a double one," 
and that the fusion of these two outpocketings resulted in the loss of the 
visceral arch which once separated them, and therefore in the loss of the 
nerve primitively related to that arch. Moreover, between the second 
head somite of van Wijhe, which extends into the mandibular arch, and 
the fourth somite, which is widely connected with the mesoderm of the 
hyoid arch, there lies the third head somite, in correlation with which 
there is no intermediate visceral arch. This somite (the 3d) lies opposite 
the posterior constriction of neuromere IV, and speaks plainly for the 
previous existence of a lost head segment, for which neuromere IV may 
once have furnished the nerve centre. Did such an arch exist, each of 
ran Wijhe’s somites from the second to the sixth, and each of the en- 
cephalomeres from III to VII would correspond with a visceral arch. 
I give a brief summary of the line of reasoning which leads me to 
believe that the significance of the hindbrain neuromeres lies in their 
primitive relationship to the visceral arches. In the young embryos 
of S. acanthias two facts, both so far as I know new, present them- 
selves. In the first place, the hindbrain neuromeres, five in number, 
are found to be successive similar thickenings of the lateral zones of 
the medulla, Secondly, from four of them, viz. III, V, VI, and VII, 
are proliferated the ganglionic cells of the four cranial nerves which in- 
nervate the first four visceral arches, viz. the trigeminus, the facialis, 
the glossopharyngeus, and the Urvagus. A clue to the significance of 
the local thickenings of the neural wall in the tract of the encephalo- 
meres is given in the fact that from those two encephalomeres which 
(in other Vertebrates as well as in S. acanthias) most closely retain 
these primitive nerve relationships, viz. III and V, emerge the fibres 
which innervate the visceral arches (primitively) related to them. The 
thickenings are the first expression of the “Kerne” (nuclei) of the 
nervous centres related to the visceral arches, and possibly also, primi- 
tively, of those related to the somites. 
1 Kupffer (’98) finds in Acipenser embryos an entodermal outpocketing or pouch, 
which soon disappears, just anterior to the hyomandibular pouch. The position of 
this pouch would identify it with the cleft whose former existence seems probable 
on the evidence given above. Houssay (’91) also recognizes in Amblystoma a vis- 
ceral cleft between the oral and the hyomandibular. 
