Charles Russell Bardeen 233 
one another and with the peripheral layer of cells. In the meshes of 
this framework the bundles of nerve-fibrils run. 
In Fig. 2 is shown the extremity of the ventral division of a spinal 
nerve of a pig 10 mm. long. This nerve was isolated and stained by 
the method described above. The advancing tip of the nerve is com- 
Fig. 2. Nerve-tip isolated from an intercostal nerve of a pig-embryo 10 mm. long. 
720 diam. 
posed of a few fibrils ensheathed by two cells. Behind this the nerve- 
fibrils and the sheath-cells rapidly increase in number. A short dis- 
tance behind the tip the nerve has been partially split during the pro- 
cess of isolation. The curving of the tip is due to mounting. 
The various constituent elements of the nerve may be isolated by teas- 
ing. The cells applied to the more compact bundles of fibrils present the 
appearance shown in Fig. 3, a. Fig. 3, b, shows a cell in the process of 
division; Fig. 3, ¢, two cells with a 
nerve-fibre passing between them; 
and Fig. 3, d, two fibres and two 
cells from the extreme tip of a grow- a 
ing nerve. With care it is possible MM. 
s 
to isolate nerve-fibres or bundles of 7 
fibrils free from any cellular cover- ” 
: g siderak : 22 + Fig. 3. Elements isolated from the ven- 
ing for considerable distances. No  traltcunkof a spinal nerve of a pig-embryo 
ae * 11 mm. long. 720 diam. 
nerve-fibres can be seen arising from 
cells within the nerves. The cells are, however, sometimes so closely 
applied to small groups of fibrils as to give rise to the appearance of 
protoplasmic continuity between sheath-cells and nerve-fibrils. This is 
especially apt to be the case in longitudinal sections. 
Fig. 4 shows a cross-section of the median dorsal branch of a thor- 
acic nerve of an embryo 18 mm. long. This nerve is similar in struc- 
