NERVE DEVELOPMENT 



113 



xist.s tin: MIMIC divergence of opinion as in the case of the motor 

 nerves, and in end< -iivmiring to decide which view has upon its side the 

 balance of probability, it is well to bear in mind similar conditions 

 to those alluded to on p. 106. Bearing these in mind, it is of interest 

 tn notice that in Lepidosiren (Elliot Smith, 1908) the process of 

 development of the olfactory nerve takes place along exactly similar 

 lines to that <>l the motor trunks. And it is significant that, in the 

 opinion o! tlmse well qualiluMl to judge (Retzius, Golgi, Ramon y 

 < 'a jal, van Gehuchten, Kolliker, Elliot Smith), this nerve has 

 advanced less from the primitive condition than has any other 

 nerve, and in its general arrangements has undergone extraordin- 

 arily little complication during 

 ontogeny. 



Already at a time when the 

 olfactory organ has not yet com- 

 menced to recede from the wall 

 of the hemisphere the olfactory 

 nerve exists as a stout protoplas- 

 mic bridge (Fig. 63, /) which 

 gradually increases in length as 

 the olfactory organ recedes from 

 the hemisphere. This observation 

 seems to indicate clearly that the 

 mode of development of the sen- 

 sory nerve-trunk is fundamentally 

 the same as that of the motor : 

 that it develops out of a pre- 

 existing protoplasmic bridge be- 

 tween centre and end-organ. 



cH. , 



I. 



FIG. 63. An early stage of the olfactory 

 nerve of Lepidosiren. (From Elliot Smith, 

 1908.) 



REMARKS UPON THE GENERAL 

 PROBLEM OF NERVE DEVELOPMENT 





<:II, lateral wall of hemisphere ; df, olfactory 

 organ ; 7, olfactory nerve. The nuclei seen in the 

 region where the olfactory nerve enters the hemi- 

 It Will be admitted by mOSt sphere belong to the olfactory bulb. 



Zoologists that we are justified 



in believing that the process of nerve -development is probably 

 fundamentally the same throughout the Animal Kingdom. It will 

 also be clear, even from the short and imperfect statement which 

 has been given here, that the detailed study of the phenomena of 

 nerve -development has led, in the minds of different observers, to 

 widely divergent conclusions as to the exact nature of the process. 

 The subject is one to the discussion of which we may devote with 

 advantage some further space. It is in itself of great embryological 

 and physiological interest. It presents many problems still unsolved. 

 And it may be taken as a type of biological controversy with which 

 it will be. to the student's advantage to become acquainted. 



In approaching the question from the present-day standpoint it 

 ap pears impossible to get round the fact that in two of the most 



VOL. II I 



