NEAL: NEKVOUS SYSTEM IN SQUALUS ACANTHIAS. 255 



h. Relations of Excephalomere YI. 

 The present structure and relations of the component parts of what I 

 reo'ard as the primitive sixth cephalic segment have been considerably 

 changed coenogenetically by the development of the otic capsule. Aris- 

 inf' from what in all probability was primitively a sensor organ of the 

 dorsal lateral line (Ayers), the great enlargement and subsequent invagi- 

 nation of this capsule bring about ontogenetically the degeneration of 

 the musculature of the 5th somite, whose cells, after assuming the 

 elongated spindle form of embryonic muscle cells, are transformed in 

 early stages into loose mesenchyma. In Ammocoetes, however, only the 

 median portion of the 1st post-otic somite disappears during ontogeny, 

 while the lateral portion forms the most anterior segment of the lat- 

 eral body musculature (muse, latei-alis capitis anterior, von KupfFer), 

 Furthermore, in Squalus the development of the otic capsule causes a 

 shifting backward of the point of exit of the fibres of the glossopharyn- 

 geus, whose ganglion cells were proliferated from encephalomere YI ; 

 moreover, the fibres of this nerve may be traced in the neural tube as far 

 forward as encephalomere YI, in which, it is my opinion, their nuclei lie. 

 The growing ganglionic Anlage of this nerve meets the mesoderm between 

 the 4th and 5th somites (Fig. 13), and I assume that it was primitively 

 related, as are the dorsal nerves of Amphioxus, to a myoseptum, i. e. the 

 one primitively between somites 4 and 5. The sensor fibres of this 

 nerve innervate the skin of the present 2d visceral cleft (Fig. 14), 

 which was, I assume, primitively inter-somitic in position and situated 

 ventral to the myoseptum between the 4th and 5th somites. Its motor 

 fibres innervate the splanchnic musculature of the present 3d visceral 

 arch, probably a primitive relation. The abducens nerve, I believe, 

 represents the primitive ventral nerve of this metamere. 



c, Eelatioxs of Encephalomere V. 



The fourth somite, the one corresponding to the fifth cephalic segment, 

 is the most rudimentary of all the cephalic somites. The phylogenetic 

 loss of its musculature and the ontogenetic dissolution of its cells into a 

 loose mesenchyma may be explained as due to the same cause as that 

 operative in the case of the 5th somite, the development of the otic cap- 

 sule. The dorsal nerve of this segment, the facialis, is inter-somitic in 

 position, occupying the constriction dividing the 3d and 4th somites 

 (Figs. 11-17), and its motor fibres innervate the (splanchnic) muscu- 

 lature of the corresponding (2d visceral or hyoid) arch. Correlated with 



