236 Growth and Histogenesis of Nerves 
limbs in several early human embryos. Grosser and Frohlich have made 
a valuable special study of the development of the thoraco-abdominal 
cutaneous nerves in man and have compared the conditions found in 
early embryos with those found in the adult. 
After pig-embryos have reached a length of from two to three centi- 
meters it is possible to get very instructive pictures by means of impreg- 
nation with gold-chloride. My best preparations have been obtained by 
placing a portion of the embryo first in lemon juice for ten or fifteen 
minutes, then in a one per cent solution of gold-chloride for an hour, 
finally reducing in a twenty per cent solution of formic acid in the dark. 
Separate layers of tissue may be isolated and spread out in glycerine, or 
the specimen may be embedded and cut. By these methods the various 
stages in the formation of the peripheral plexuses may be most readily 
followed. 
From the primary cutaneous rami extending toward the skin from 
the main nerve-trunks in the deeper parts branches are given off which 
run in various directions parallel with the epidermis but some distance 
below it. From these branches the main subcutaneous nerve-plexuses 
directly arise. The formation of these subcutaneous plexuses seems to 
be due to the tendency of branches sent by two nerves into a common 
region to be attracted toward the same area and to fuse into a common 
trunk on reaching it.” In Fig. 5 the larger, darker nerves here repre- 
sented as the most superficial, form a portion of the main subcutaneous 
plexus arising from branches of the lateral rami (Tr.) of the dorsal 
divisions of two thoracic nerves of a pig embryo 4 cm long. While 
this plexus is being formed branches are given off from the nerves 
forming it. These extend towards the epidermis, just below which 
another plexus with finer meshes arises. In Fig. 5 this second plexus 
is shown in process of formation. The nerves entering into this latter 
plexus stain much lighter than the main nerve-trunks. As development 
proceeds from the stage illustrated in Fig. 5, more and more branches 
are given off from the plexuses there shown, finer plexuses are formed, 
and ultimately nerve-fibrils are distributed to the various end-organs and 
structures characteristic of the skin and subcutaneous tissue. The de- 
tails of these latter processes I have not attempted to follow. 
? Nerves growing into a given region from two or more directions do not always 
thus anastomose to form plexuses. Thus Mertens has shown that although the lateral 
cutaneous branches of the fourth and fifth intercostal nerves supply overlapping 
areas, anastomoses between the nerve branches revealed by dissection are infrequent. 
A bundle of fibres constituting a nerve may cross over or through another nerve 
without real anastomosis of the two trunks. 
