NEAL: NEKVOUS 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 filn-es^ 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 trush 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 muse, 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 Piatt 

 ('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 nervfe fibres. When the only sec- 

 tions I possessed w^ere of embryos killed with corrosive-sublimate acetic, 

 and stained with carmine or hsematoxylin, the evidence seemed to me 

 confirmatory of the view of Froriep ('91), viz. that the trochlearis is 

 differentiated from mesenchymatous cells in situ. But better methods 

 of preparation have taught me to distrust that evidence, and the results 

 appeared to me too distinct^ contradictory to the later histological 



cross each other in growth, since the direction of their growth would thereby be 

 unchanged. I assume that it is easier for a nerve fibre to grow in a direct line 

 than to bend 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 muse, obliq. sup. and the trochlearis. 



