122 Experiments on the Development of Peripheral Nerves 
in which the spindle shaped sheath cells were eliminated by the removal 
of their source, at least their principal source, in an early embryonic 
stage, before nerves of any kind are visible. The experiment consisted 
in removing the ganglion crest. ‘This was done by cutting off a thin 
strip from the dorsal side of the body of embryos (Rana esculenta) from 
2.7 to 3 mm. in length (Fig. 1). Since this operation removes the source 
of the spinal ganglia also, the embryos develop without sensory nerves 
and ganglia, but the motor nerves do develop, and instead of being cellu- 
lar in structure, as is the case in normal snecimens (Figs. 2 and 3), they 
consist of naked fibers, which can be traced in a number of cases as far as 
the extreme ventral part of the musculature, 7. e., as far as the nerves 
extend in the adult organism (Fig. 4).° 
The first experiments were made upon fana esculenta; they have since 
been confirmed upon the embryos of two American species, RP. sylvatica 
and R. palustris. 'These experiments concerned only the spinal nerves. 
a b 
Fig. 1. Profile view of frog embryo (Rana esculenta, 2.7 mm. long) at the 
stage of operation; the line (ab) indicates the incision. 
Last season an attempt was made to corroborate the results in cranial 
nerves. For this purpose the cranial ganglia, the skin covering the side 
of the head, and the dorsal part of the brain were excised from one side 
of the embryo before closure of the medullary folds. With one exception 
2 The experiment was based upon the assumption that the ganglion crest is 
the source of the sheath cells. The result of the experiment proves this to be 
true, as far as the early stages of development are concerned. In certain 
lower vertebrates, particularly in Elasmobranchs, it has been shown that large 
numbers of cells are given off from the ventral part of the medullary cord, 
wandering out along the motor roots of both the cranial and spinal nerves, 
and giving rise at least in part to the nerve sheaths. The literature bearing 
upon this subject has recently been considered by Neal (Mark Anniversary 
Volume, New York, 1903). In the frog such cells are not given off until the 
yolk is nearly gone but after this period cells do wander out singly along the 
motor roots, and in these features the frog embryo resembles closely the 
salmon (Harrison, Archiv f. mikrosk. Anat., Bd. 57, 1901). The cells do not, 
however, begin to come off until the motor nerves are well developed and 
have reached the extreme end of their course. Thus it happens that in the 
experiment the nerves are developed without sheath cells. 
