14 Proceedings of the Royal Physical Society. 



(3) The at-first naked and non-nucleated nerve trunk acquires a sheath, 

 the heavily yolked nature of whose protoplasm demonstrates it to be of 

 mesenchymatous origin. This sheath also first appears round the nerve 

 trunk peripherally, so that a central origin by emigration from the spinal 

 cord is absolutely excluded. 



Pig. 4. 



Similar section ; stage 29 + . 

 The heavily yolked protoplasm has now spread itself out along the 

 course of the nerve trunk so as to form a definite though still very thick 

 sheath. In later stages of development the yolk is gradually used up and 

 the sheath protoplasm is reduced to a thin layer. 



11. , Nerve trunk ; sh. , Sheath. 



The observations summarised may, I think, be taken as definitely 

 established, the phenomena described being relatively plain and distinct 

 in Lepidosiren. Here, for example, there is no room for doubt regarding 

 the mesenchymatous origin of the protoplasmic sheath, whereas in such 

 forms as Elasinobrauchs, where it possesses no conspicuous histological 

 characters, the appearances seen in sections may be interpreted, according 

 to the standpoint of the observer, in totally different fashions. 



If these observations are correct, it must be admitted to be probable 

 either that (1) they illustrate the fundamental features of nerve development 

 of the Vertebrata ; or that (2) Lepidosiren developes its motor nerve trunks 



