THE GROWTH AND HISTOGENESIS OF THE CEREBRO- 
SPINAL NERVES IN MAMMALS. 
BY 
CHARLES RUSSELL BARDEEN, 
Associate-Professor of Anatomy, the Johns Hopkins University, Baltimore. 
Wir 15 TEXT FIGURES. 
In the following article are given the results of a study of the histo- 
genesis and the general mode of growth of the cerebro-spinal nerves in 
mammals. Owing to the ease with which the material could be obtained 
pig-embryos have been those chiefly used, but in addition embryos of 
man, guinea-pigs and mice have been studied. In all of these animals 
the development of the nerves seems to be essentially similar.’ 
I. OUTGROWTH OF THE CEREBRO-SPINAL NERVES. 
In the mammals the development of the peripheral nerves, with the 
exception of the optic and olfactory, begins by an outgrowth of naked 
processes from cells lying in the motor root-zone of the central nervous 
system and in the sensory ganglia. The naked processes belonging to a 
given cerebro-spinal nerve are usually grouped in bundles which extend 
out separately into the mesenchyme surrounding the central nervous 
system but which soon are collected into a common nerve-trunk. ‘This 
method of development has been clearly described by His in the human 
embryo, and can readily be verified in any of the mammals more com- 
monly studied. If a pig embryo 8 mm. long, for instance, be fixed in 
Zenker’s fluid and hardened in alcohol, the thoracic nerves may readily 
be dissected out. At this period bundles of fibres from the motor-root 
zone and from the spinal ganglion of each thoracic segment have be- 
come grouped into a spinal nerve which extends towards but does not 
enter the body-wall. One of these nerves, with its sensory-root and 
ganglion, motor-root, and main-trunk intact, may be isolated, together 
with a slight amount of the surrounding mesenchyme, stained in Dela- 
!The literature on the general subject of the development of the peripheral nerves 
in vertebrates has been recently reviewed by Harrison, Dohrn, Firbringer and 
Nussbaum, each from a different standpoint. 
