242 Growth and Histogenesis of Nerves 
The development of this sensory and vaso-motor apparatus can only be 
followed with satisfaction when better methods for differentially staiming 
developing nerves have been devised. Methylene-blue, which is so good 
for adult tissues, seems to act much less specifically on developing nerve- 
fibres. Gold-chloride used on fresh tissues is excellent for the study of 
the grosser tissue relations, but the tissues are so altered by the acids 
used in reducing the gold that the finer histological details are many of 
them lost. The Apathy method of using gold-chloride on fixed tissues 
does not seem to give good results with mammalian embryos. In young 
embryos Congo-red gives an excellent stain for nerve-fibrils, but it is not 
a stain sufficiently specific for the complex tissue relations of later em- 
bryonic development. The changes, however, taking place in nerve- 
trunks may be traced both in teased specimens and in sections, by means 
of the usual methods of technique, and the development of the nerve- 
endings on the striated muscle-fibres may be followed, although less per- 
fectly than one might wish, by the use of the gold-chloride method com- 
bined with other methods of technique. We shall now consider certain 
of the histogenetic phenomena disclosed by these methods. It is con- 
venient to describe first the processes connected with the forward growth 
and secondly those connected with the internal differentiation of the 
nerves. ‘ 
III. HistoGENESIS IN THE DEVELOPING NERVES. 
a. Processes Taking Place at the Growing Tip. 
A description has above been given of the histological structure of the 
primary embryonic nerve-trunks. Each nerve is composed of bundles 
of embryonic fibres and fibril-groups. The nerve as a whole is fairly 
completely ensheathed, while the constituent fibre-bundles are supported 
and partially ensheathed by anastomosing cells. When a nerve-trunk 
gives rise immediately to large primary branches, as is the case -with 
the spinal and most of the cranial nerves, these branches are similar in 
structure to the primary nerve-trunks. The nerves which arise by 
direct forward extension of the primary branches, such, for instance, as 
the ventral tip of each intercostal nerve and the nerves which arise from 
the fusion of two or more primary branches, such as the nerves arising 
from the brachial and lumbo-sacral plexuses, send forward a few fibrils 
closely accompanied by, perhaps preceded by, sheath-cells. Behind this 
tip the ingrowth of bundles of fibrils and the multiplication of these 
fibrils by branching causes the nerve-trunk rapidly to increase in thick- 
ness toward the central nervous system (see Fig. 2). When the nerves 
