488 



SPECIAL HISTOLOGY. 



exception of the extremities of the nerves, they are developed 

 from fusiform nucleated cells, which are nothing else than 

 modifications of the primordial formative cells of the embryo, 

 and are conjoined into pale, flattened, elongated, nucleated 

 tubules or fibres 0-001 — 0'003"' broad. Now, at first the 

 nerves consist only of fibres of this kind, and of the rudiment 

 of the neurilemma, being grey or dull white, like the sympa- 

 thetic filaments; subsequently, in the human embryo at the 

 fourth or fifth month, they always assume a whiter colour, and 

 the proper white or medullary substance continues to be more 

 and more developed in them. Of the three possible modes of 

 development of this substance propounded by Schwann, one 

 only, in the present state of things, can come into question, 

 that namely, as to whether the medullary sheath is a structure 

 deposited between the membrane and the contents of the 

 embryonic nucleated fibres; in which case the contents of the 

 latter would become the axis-fibre. But besides this, the 

 medullary sheath may originate in what did not occur to 

 Schwann, viz., a chemical metamorphosis of the external por- 

 tion of the contents of the embryonic-fibres; and the axis-fibre 

 may be only the remainder of those contents which has not under- 

 gone a fatty metamorphosis. It is difficult to determine which 

 of these two views is correct. Direct observation shows only 

 this much, that the contents of the pale embryonic fibres 



invariably, by degrees, obtain dark contours, and ultimately 

 present the aspect of a true dark-bordered fibre, whilst it 



Fig. 165. 1, two nerve-fibres from the ischiatic nerve of a sixteen-weeks' embryo ; 

 2, nerve-tubes from a newly-littered Rabbit; a, their sheath; b, nucleus; c, medul- 

 lary sheath : 3, nerve-fibre from the tail of the Tadpole; a, b, c, as before; at d, the 

 fibre retains its embryonic character ; the dark-bordered fibre shows a division. 



