I. INTRODUCTION. 



The present study of the histogenesis of spinal ventral nerves has grown out 

 of an attempt to determine the morphology of the eye-muscle nerves. If the eye- 

 muscle nerves are morphologically comparable to spinal ventral nerves, with which 

 they have been so often compared, their mode of development should be similar. 

 If differences appear these should be explicable on the ground of specialization. 

 The necessity of embryological confirmation of morphological conceptions is so gener- 

 ally recognized that it is not a little surprising to find that in most discussions of 

 the morphology of the eye-muscle nerves their actual histogenesis has been generally 

 disregarded. As Minot ('96) says with characteristic clear-headed incisiveness, 

 "the attempt has been made to solve the most difficult questions of the morphology 

 of cranial nerves without answering the inconvenient question of nerve-fibres and 

 their sheaths." The need for renewed investigation of the histogenesis of not only 

 the eye-muscle nerves in Selachii, but also of the spinal ventral nerves with which 

 they have been compared, is known to all familiar with the literature dealing with 

 this problem. Not even the fundamental question of ventral-nerve histogenesis, 

 viz., whether fibres grow as processes of medullary cells or by the fusion of cell-chains, 

 which has been settled for Amniota in favor of the "process theory," can be regarded 

 as settled for Selachii. Beard ('96, p. 395) even goes so far as to say that "any one 

 who has devoted time and pains to the study of Elasmobranch embryology could 

 not but be very curious to see a work on this group, proving that the nerves of these 

 fishes always arise as processes of neuroblast cells." Beard, at least, has the right 

 to claim, in view of the differences in the results of investigators, that the question 

 of nerve histogenesis for the group of Selachii "is still an open one." 



A priori reasoning as well as observations have led to divergent opinions. His 

 ('89), for instance, writes that no clear-headed investigator will maintain that the 

 mode of nerve formation in selachians is fundamentally different from that in other 

 vertebrates. If in higher forms ventral nerves are fibrous outgrowths they must be so 

 in lower forms. Yet Gegenbaur ('98, p. 722) on a priori grounds reaches a very dif- 

 ferent conclusion. He says that in invertebrates, cell nuclei appear in the course 

 of the nerves, from which we may infer that cells participate in the formation of nerve- 

 fibres. In the vertebrates also similar relations appear in the branches of both 



293 



