NERVOUS FIBRES. 148 
explain the relation which they bear to the cells. Remak! 
describes the early condition of the nerves in the following 
manner: ‘The substance of the cerebro-spinal nerves of the 
rabbit, in the third week of embryonal existence, consists of 
corpuscles, some of which are irregularly spherical, others 
slightly elongated, having a very delicate filament adhering 
to them; they are mostly transparent, and arranged in rows 
without, however, presenting any distinctly perceptible fibrous 
structure.” And l. c. page 153, he says, “ A structureless and 
general globular mass is the original form, from which the 
primitive fibres of the cerebro-spinal nerves are developed. 
These primitive fibres are at first varicose, and contain no 
medulla; most of them pass into the cylindrical form, through 
the intermediate stage of transitional fibres.” 
I have investigated the development of nerve in the feetal 
pig. The nerves of the foetus have not the shining white 
colour, presented by those of the adult animal, but are gray 
and transparent, and the younger the embryo the more strik- 
ing are these appearances. We are, therefore, quite prepared 
to find that microscopic imvestigation shows the white sub- 
stance of the fibres to be less perfectly or not at all developed. 
If a nerve, taken from a foetal pig of about six inches in length, 
be spread out, in the usual mode of preparation by tearmg it 
under water, some fibres are seen which very much resemble 
those of the adult animal, and which are furnished with 
outlines almost as dark. The greater part of the substance, 
however, does not form connected fibres, but consists of separate 
round globules, or more or less long, irregular little cylinders, 
arranged with their long axes in the direction of the course of 
the nerves, having outlines, however, quite as dark as those of 
the nervous fibres. These appear to be what Remak refers 
to in the description previously quoted. In addition to them, 
however, a substance of quite another appearance is seen, 
which has not the dark outline, does not appear pellucid but 
granulated, and in which the cell-nuclei are distinctly recog- 
nisable. 
When the other constituent parts predominate, the nuclei 
1 Miller’s Archiv, 1836, p. 148. Respecting the microscopic structure of the 
brain and spinal cord of the foetus, see Valentin, Entwickelungsgeschichte, p. 183. 
