294 THE DEVELOPMENT OF THE VENTRAL NERVES IN SELACHII. 



medullated and non-medullated nerves. In the case of the former the medullary 

 sheaths and in the latter the axis cylinder, are directly enclosed in a delicate nucleated 

 membrane (neurilemma). This neurilemma has been regarded as something added 

 to the nerve from the connective tissue. Of this the proofs are as yet uncertain, 

 and it is more probable that the nuclei of the neurilemma belong to cells which have 

 served in the formation of the fibre. Thus a priori reasoning works both ways, 

 depending on the point of departure. 



Observations also on the histogenesis of ventral nerves differ. Concerning their 

 histogenesis in the Amniota there is practically a consensus of opinion in favor of 

 the "cell-process" theory. There is still debate concerning nerve histogenesis in 

 the Anamnia. His ('89), von Lenhossek ('92), and Neal ('98) claim to have discovered 

 the same mode of ventral-nerve histogenesis in Anamnia as in Amniota. Yet the 

 latest investigation of nerve histogenesis in Anamnia, viz., that of Raffaele (:00), 

 confirms the "cell-chain theory" supported by Balfour, Dohrn, Beard, Gegenbaur, 

 and others. Von Lenhoss6k ('98, p. 114), however, holding with His and others, 

 that there is but one mode of nerve development for all vertebrates, easily disposes 

 of such results as those of Dohrn and Beard by claiming that their technical methods 

 are unsuited for purposes of nerve study. They make the mistake, says von Len- 

 hossek, of attempting to solve one of the most difficult histological problems on the 

 basis of an inadequate method, and again the second greater error that, when fine 

 and more suitable methods for the investigation have been invented, they cling 

 to old methods and disregard the more recent discoveries, because these cannot be 

 made by the older methods. While it must be admitted that this criticism is in a 

 measure just, nevertheless, in view of the fact that neither von Lenhossek nor any 

 one else who has applied the Golgi method — which von Lenhoss6k evidently regards 

 as an eminently suitable method — to the study of nerve histogenesis has been able 

 to effect a successful impregnation of nerves in the early stages of development con- 

 cerning which the results of investigators differ most, the criticism does not really 

 invalidate the results based upon the older — "borax-carmine" — methods. In fact, 

 "borax-carmine" preparations will give positive results in early stages of nerve 

 development where the Golgi method completely fails. These are the very stages, 

 moreover, in the development of the anamniotic embryo, which afford evidence of 

 the migration of medullary elements into ventral nerves and of the much-disputed 

 cellular nature of embryonic nerves. If the claim should be made that it does not 

 matter whether the development of neuraxones is studied in earlier or later stages, 

 inasmuch as it may be safely assumed that neuraxones grow in all stages alike, the 

 reply may be made that the truth of this assumption is not certain. At any rate, 



