THE ItOOT-SHEATH IN HEDGEHOG-Sl'INES. 127 



XII. 



ON THE STRUCTURE OF THE ROOT-SHEATH 

 IN HEDGEHOG-SPINES. 



BY R. BROOM, M.B., CM., B.SC. 



[Read 29th October, 1886.] 



The main points of difference between the growing 

 and fully-developed Hedgehog-Spine have long been 

 known. But in recent years, with the exception of 

 Lwoff,* I am not aware of any investigator having 

 made a careful study of the details of structure of 

 the spine and its sheath ; and Lwoff, while he has 

 made an examination of all the parts connected 

 with the spine, has devoted most of his attention to 

 the structure of the elements of the spine itself. 



Whether a spine is an enormously developed hair, 

 or a hair a very delicate and elongated spine, I 

 shall not discuss at present. There can be no doubt 

 that the two are homologous structures ; but to see 

 the exact homology it is necessary to compare the 

 growing spine with the growing hair. In the fully 

 grown spine, as there is no longer anj^ need for the 

 inner root-sheath, this undergoes degenerative 

 changes, and the structure can no longer be made 

 out; but in the growing spine the layers of the 

 root-sheath are seen to perfection, and, owing to 

 the inner sheath being developed to a much greater 

 extent relatively than in the hair, we have in the 

 Hedgehog special facilities for the study of the 

 structure of the various layers. 



On examining a longitudinal section of the grow- 

 ing spine with its sheaths, what strikes one most is 

 the absence of a differentiated layer corresponding 



Bull. Soc. Imp. Mosc, T. llx. (18&4), p. 141. 



