234 Growth and Histogenesis of Nerves 
ture to the main ventral branch of an embryo of 10 mm. About the 
periphery of the nerve cells may be seen which give rise by flattened 
anastomosing lateral processes to an inclosing membrane. Within the 
body of the nerve many cells may be seen giving rise to processes which 
anastomose with one another and with processes arising from the mar- 
ginal cells. On staining in hematoxylin and then in the Van Gieson 
mixture the processes of the marginal cells and of the internal cells 
take a purplish tint, while the cross-sections of the nerve-fibrils appear 
orange in color. A slight bluish tint, often taken by the areas inter- 
vening between the nerve-fibrils and the sheath-cells, indicates that some 
substance is present there. This corresponds to the homogeneous mate- 
rial which Vignal has described as constituting the stroma of the bun- 
dles of nerve-fibres. 
Fic. 4 Cross-section of the median ramus of the dorsal division of a thoracic 
nerve of a pig-embryo 18 mm. long. a, b, ¢ branches arising from the main trunk. 
720 diam. 
The processes taking place after the formation of the primary em- 
bryonic nerves may be conveniently divided into two groups, those un- 
derlying the distribution of nerves during development and those under- 
lying the histogenetic changes taking place within the growing nerves. 
We shall now consider each of these groups in turn. 
IJ. Nerve Distrrpution DurInGc DEVELOPMENT. 
After the formation of the primary trunks of the cerebro-spinal 
nerves growth towards the areas which they are to supply becomes very 
active. In case of the sixth cranial nerve in man the nerve-trunk 
extends directly to the anlage of a single peripheral organ. In most 
instances, however, the conditions of nerve growth are far more complex. 
The primary nerve trunks give rise to primary branches each of which 
is destined for a more or less complex area of the body. Thus the fifth 
cranial nerve gives rise to its three chief branches, the cervical and 
