389 
those two encephalomeres which most closely retain these primitive 
nerve relationships (in other Vertebrates as well as in 8. acanthias), 
viz. III and V, emerge the fibres which innervate the visceral arches 
primitively related to them. The shifting of the point of exit of the 
roots primitively related to encephalomeres VI and VII may easily be 
explained by the crowding caused by the ear capsule. The relation of 
the nerves gives a clue to the significance of the local thickening of 
the neural wall in the region of the encephalomeres. The thickenings 
are an expression of the ‚‚nuclei“ (Kerne) of the nerveselated to the 
visceral arches. Now since four encephalomeres, viz. III, V, VI and 
VII, are clearly related to four visceral arches, we should expect 
the omitted encephalomere, viz. IV., to have been primitively related 
to a visceral arch. The fact that an encephalomere with a local 
thickening exists unrelated to a visceral arch may seem at first sight 
a disproof of the hypothesis made. On the other hand, that a vis- 
ceral arch has been present in the region of this encephalomere, has 
been considered probable by van WisHE (82), Miss PLarr (91) and 
HoFFMANN (94), as the result of their studies on Mesoderm. The 
evidences from the study of neuromerism and mesomerism seem 
mutually confirmatory and to the effect that a visceral arch has been 
lost during phylogeny in the region of van Wwume’s 34 somite +). 
Granting that this encephalomere by virtue of its local thickening 
furnishes evidence of a lost visceral arch, then we have a correspond- 
ence of neuromeres, mesomeres and branchiomeres from VAN WIJHE’S 
2nd to his 6th somite. The evidence offered by the local thickening 
in the region of encephalomere IV tends to make the assumed 
correspondence between primitive mesomerism and branchiomerism 
more probable. The relations of ectodermal, mesodermal and ento- 
dermal segments, as I find them in S. acanthias, may be summarized 
in the following table. 
Encephalomeres I II | Ill | IV V VI VII 
Somites A 1 2 3 4. 5 6 
Nerves, Dorsal 2 Ch. 2 V lost VII IX IX | 
oF Ventral lost III IV VI VI VI VI 
Visceral clefts Hyp. Mouth lost 1 2 3 
» arches lost lost 1 lost 2 3 4 
1) Since the facialis is the proper nerve of the hyomandibular cleft, 
it follows that the lost nerve and cleft lay anterior to the present 
hyomandibular cleft. For its hypothetical relation, see the accompanying 
table. 
