vii STRUCTURE AND ORGANISATION OF NERVE 45 



(Kolliker, Rouget, Hensen). Hensen found that these nerves at 

 first consisted of fine shining forked threads without nuclei ; later 

 on single nuclei appeared, at first near the body-axis, and subse- 

 quently in the terminal ramifications also. These cells undoubtedly 

 belong to the connective substance, from the fusion of which 

 arises the sheath of Schwann. Neither medullary sheath nor 

 sheath of Schwann appears to develop continuously at all points 

 along a nerve-fibre. 



It is known that in the central nervous system (brain and 

 cord) the fibres of the pyramidal tract become gradually invested 

 with medullary sheaths, in the direction from parent cells to 

 spinal cord, and the same is stated by Kolliker of peripheral 

 nerves, where the development of medulla is directed from trunk 

 to periphery. Kolliker disputes the assumption of Hensen that 

 the medulla originates in the form of single drops, since he 

 observed on the tadpole " that it appeared ab initio as a coherent 



FIG. 155. Two medullated nerve-fibres of Palaemon squilla. (Retzius.) 



tube, which gradually acquired the dark contour, the transition 

 from pale to dark-walled fibres being thus imperceptible." 

 This seems to occur first near the nuclei of Schwann, so that 

 medullary segments are formed, separated by longer non-medullated 

 tracts corresponding with the constrictions of Ranvier. 



The presence of true medullary sheaths in certain nerves of 

 invertebrates has frequently been asserted, but is under all 

 circumstances a rare occurrence. Without entering into the older 

 researches, we may quote the investigations of Retzius on Palaemon 

 squilla, and of Friedlander in Annelids (Mastobranclms, Lumbricus), 

 as proving the existence of medullated nerve-fibres in invertebrates. 



The nerve-fibres of Palaemon present the most exact structural 

 conformity with the medullated nerves of vertebrates. In these, 

 Retzius was able by the silver method, and also by methylene 

 blue, to show characteristic figures corresponding with Ranvier's 

 crosses, as well as Frommann's lines, corresponding with the 

 constrictions, and appearing at definite and regularly recurring 

 intervals (Fig. 155). Between the nodes there is a long oval 



