152 PROTOPLASM 



objects, in definite tracts one behind the other, from which 

 arrangement arises the fibrous structure. 



I have observed very plainly exactly the same structure 

 of the axis-cylinder in cross sections through the ischiadic 

 nerve of the rabbit, prepared in a corresponding manner. 

 Fig. 2, Plate X., gives a figure of such a preparation drawn 

 with a camera lucida, taken from a section as thin as possible, 

 which only showed fragments of the nerve. 



Since in cross sections of the ischiadic of the frog some 

 peculiarities can also be observed in the sheath of Schwann, 

 I will describe them briefly. The sheath always stains very 

 intensely with Delafield's hsematoxyUn, while the axis- 

 cylinder, on the other hand, remains very pale. It can now 

 be plainly observed that the substance of the sheath has a 

 meshwork structure, consisting, in fact, of a single layer of 

 meshes, the walls of which are directed vertically to the 

 surface of the sheath (Plate IX. Fig. 4, a-c). At a spot 

 where a nucleus is lodged in the sheath it becomes thickened, 

 increasing at the same time to two layers of meshes, one 

 of which then encloses the nucleus internally, the other 

 externally, so that it comes to lie in the protoplasm. We 

 thus find exactly the same relations as have been already 

 described for the cells of the capillaries, and for the peculiar 

 connective tissue cells of the ischiadic nerve. 



As I did not make a special study of the substance of 

 the medulla, I will merely remark briefly, that a net-like 

 meshwork can be made out in it very ■well, both in entire 

 isolated fibres, after treatment with picro-sulphuric-osmic 

 acid, and in longitudinal and transverse sections of them. 

 Just in the same way I have always found a dark sheath very 

 distinct round the axis-cylinder, as is especially well shown 

 in Photograph XIV. As to the relations of the two 

 sheaths to one another, and to the medullary substance, I 

 can only express an opinion by the way, that these three 

 parts may well form together a single whole. 



