458 NOTES AND MEMORANDA. 



peculiar appearance of the nerve-medulla is easily recognised 

 without the action of any reagent. However, the charac- 

 teristic brownish-black colour given to it by osraic acid dis- 

 tinguishes it with great precision. 



At certain points in the course of the nerve are seen areas, 

 more deeply stained by osmic acid than the rest of the nerve. 

 The parietal nuclei are always seated at the centre of this 

 deeply-stained substance, as if this were deposited around 

 the nuclei. Here we have the commencing formation of 

 nerve-medulla, which appears on the bundle of fibrils in the 

 shape of interrupted masses, the interruptions corresponding 

 to the intervals between two nuclei. 



The effect of this process in a meduUated nerve-fibre that 

 has reached a certain stage of development is very remark- 

 able. The nerve-medulla is deported on a bundle in such a 

 way as to form segments, which are more or less regular and 

 limited on both sides by a point where the medulla is wanting 

 (a constriction of Ranvier). 



Towards the middle of each segment is seen a nucleus. 

 At this stage, then, the embryonic nerves show the charac- 

 ters described by Ranvier, Axel Key, and Retzius. The 

 author finds that, at the level of each constriction a principal 

 trunk gives off a collateral branch, which also behaves in a 

 similar way. 



When these researches were being made this peculiarity 

 had not been described by other workers in the same field, 

 but a little later Rouget also drew attention to this fact in a 

 memoir on the development of nerves (' Archiv. de Phys.,' 

 Feb., 1876). 



A bundle of fibrils seldom becomes medullated throughout 

 its whole circumference. Asa rule, some portions at the side 

 of a bundle of medullated fibres preserve their embryonic 

 character. This fact, first noticed by Rouget, is explained 

 by him as follows : 



" The multiplication of nerve-fibres consists in a process 

 of cleavage. The nucleus divides in the direction of its long 

 diameter, and simultaneously the whole fibre splits longitu- 

 dinally into two. 



" In a system of two fibres united together, one undergoes 

 a progressive development, becoming medullated, while the 

 other, preserving its embryonic character, persists as a pale 

 fibre, and provides for the multiplication of the nerve- 

 fibres." 



Leboucq considers, however, that the fact that collateral 

 branches arise from the medullated fibres at the level of the 

 constrictions of Ranvier shows that these same medullated 



