542 LOCV. [Vol. XI. 



mere, but in the brain the primitive relations have been greatly 

 modified or obliterated. There is not sufficient evidence to 

 show what might have been the primitive arrangement in that 

 region. Of course, we bear in mind that the sensory fibres 

 grow centripetally, but if they appertained to a particular seg- 

 ment, we might make a tentative estimate of numerical relations 

 as follows : — 



I. First Neuromere of Fore-brain, Olfactory. 

 II. Second Neuromere of Fore-brain, Optic. 



III. Third Neuromere of Fore-brain, possibly nerve to Pineal Sense- 



organ ? 



IV. First Neuromere of Mid-brain, Oculo-motor. 

 V. Second Neuromere of Mid-brain, Trochlearis. 



It is easier to assign relations for the segments of the 

 hind-brain. In my assignment I agree substantially with 

 Hoffmann. 



VI. First Neuromere of Hind-brain, anterior root of Trigeminus. 

 VII. Main root of Trigeminus. 



VIII. Third Neuromere of Hind-brain; no nerve root, at least in 

 early stages. 

 IX. Facialis. 

 X. Auditory. The roots of the Facialis and the Auditory arise 



separately in Squalus acanthias. 

 XI. Glosso-pharyngeal. 

 XII, XIII, XIV. Roots of the Vagus. 



At the time of its first appearance the auditory saucer lies 

 in front of the tenth neuromere, but is soon shifted backwards 

 opposite the eleventh. 



Minot has directed attention to the fact that Miss Piatt and 

 McClure ignored the difference between ganglionic and medul- 

 lary nerve-fibres, and this is important ; but the ganglion 

 ridges are divided in the same way as the neural tube, and we 

 may speak of the neuromeres as including the segments of the 

 ganglion ridge. 



The relation of the nerve-fibres to the neuromeres will 

 be considered more in detail under the heading of The 

 Nerves, 



