NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 237 
receive the name of ganglion, there is no evidence at this or later 
stages. The last traces of scattered groups of neural-crest cells found 
in some (not all) embryos of earlier stages (17 mm.) have been 
lost. 
In embryos of 21 mm. some of the distal fibrils of the nerve appear 
to have united with migratory cells from the r. ophth. sup. trigeminus, 
a process in my opinion comparable with that which takes place in the 
growth of the oculomotorius, At least, in embryos of 21 or 22 mm. the 
distal portion shows nuclei in relation with the nerve fibres, whereas 
proximally no nuclei are seen. In still later stages the nerve has a dis- 
tinctly cellular appearance throughout its length. The nuclei are, how- 
ever, seen in thin sections to be peripheral in relation to the nerve fibres, 
as in the oculomotorius (Figure I), The evidence of anastomosis of the 
fibres of the trochlearis and the r. ophth. sup. trigemini I consider 
very doubtful. During development the loose brush of fibres at the 
distal termination of the trochlearis becomes united into a compact nerve 
stem. It has, therefore, seemed to me that the primary widely spread 
brush of nerve fibrils may be explained on the ground of advantage 
gained in seeking the terminal organ, the musc. obliqu. superior, 
The phenomena observed by me during the development of the trochle- 
aris are seen to correspond very closely with those observed by Miss Platt 
(91). To her, as to me, the trochlearis first appears as a fibrillar process 
from the dorsal wall of the brain. But while she interprets the evidence of 
cellular growth toward the advancing end of the nerve as of morphologi- 
cal or phylogenetic significance, I am unwilling to give it such interpre- 
tation, since I find that these nuclei have nothing to do with the nerve 
proper. In my opinion, it is probable that they become converted into the 
nuclei of Schwann’s sheath, an opinion which seems confirmed by their 
peripheral position in relation to the nerve fibres. When the only sec- 
tions I possessed were of embryos killed with corrosive-sublimate acetic, 
and stained with carmine or hematoxylin, the evidence seemed to me 
confirmatory of the view of Froriep (91), viz. that the trochlearis is 
differentiated from mesenchymatous cells ¿n situ. But better methods 
of preparation have taught me to distrust that evidence, and the results 
appeared to me too distinctly contradictory to the later histological 
cross each other in growth, sinc. the direction of their growth would thereby be 
unchanged, [assume that it is easier for a nerve fibre to grow in a direct line 
than to pend back and reverse the direction of its growth. The possibility even of 
a primary connection of muscle and nerve appears to me to be excluded in the 
case of the musc. obliq. sup. and the trochlearis. 
