Charles Russell Bardeen 249 
to that previously described in page 234.  Kolster has described the inva- 
sion of cells of the sheath in among the nerve-fibrils which it surrounds 
in the nerves of Salmo trutta. 
During the growth of the nerves the elementary bundle or bundles 
of fibrils of which it is composed become further broken up into second- 
ary bundles by the invasion of fibrous tissue from the investing sheath. 
Thus, each intercostal nerve of the pig is at first composed of but a 
single funiculus, but later it becomes divided into two main bundles of 
fibrils, and these again are further subdivided. Fig. 13, a, shows a cross- 
section of an intercostal nerve 
of an embryo 8 ecm. long. 
Above is a rather small com- 
pound bundle of fibrils, and be- 
low is a much larger one. Each 
of these is in turn divided into 
several funiculi. The septa sepa- 
Fia, 12. Fie. 13. 
Fig. 12. Cross-section of the median branch of the dorsal division of a thoracic 
nerve of a pig embryo 14 mm. long. 720 diam. 
Fic. 15. a, Cross-section of an intercostal nerve of an embryo 8 cm. long; 3, 
cross-section of the bundle of nerve-fibres designated ‘‘1’’ in ‘‘a’’; ¢, An isolated 
cell corresponding to the cells shown in section in the bundle of nerve-fibres in ‘*b”’; 
d, longitudinal section through one of these cells; e, small portion of the membra- 
nous sheath surrounding the bundle of fibres shown in ‘“‘b.”? a, 76 diam.; 0, c, d and 
Y 
e 720 diam. 
rating the funiculi last formed are delicate. This is shown in the sep- 
tum at the lower margin of the funiculus pictured in Fig. 13, b, an 
enlargement of “1” in Fig. 13, a. But these septa are always lined 
toward the funiculus by a layer of flat, anastomosing cells. In Fig. 13, ¢, 
is shown a portion of such a membrane isolated from a funiculus of an 
intercostal nerve of a pig 8 em. long and stained in hematoxylin and 
safranin. 
The invasion of tissue which breaks up the elementary bundles into 
