142 Victor E. Emmel. 
purely supportive fibrils differentiate within the epidermal cells, 
but these structures are secondary in their relation to muscle 
attachment. 
2. Nerve Fibers.—In studying the regeneration of nerve fibers 
we are at once involved in the intricate questions of the origin of 
the neurilemma, and of the axis cylinder with its neurofibrille. 
Complete agreement regarding ectodermal or mesodermal origin of 
these structures has not yet been attained. Regarding the neuri- 
lemma, for example, the conclusions of one group of investigators 
may be summarized in the statement that sheath cells “are true con- 
nective tissue cells from the locality through which the nerve fiber 
has passed in its development” (Dahlgren and Kepner, ’08, p. 188). 
On the other hand, the ectodermal origin of the sheath cells appears 
to be strongly supported by experiments in which it has been shown 
that in the tadpole at least ‘the source of the sheath cells, both of the 
motor and sensory nerves is in the ganglion crest’? (Harrison, 08, 
p- 393). Although the present observations on the lobster are, on 
account of a lack of material especially prepared for neurological 
study, not sufficiently extensive to render them very important in 
relation to these fundamental problems, still they may be of some 
value. 
In early stages of the regenerating limb bud there is found a cord 
or column of cells extending from the tip of the bud to the end of 
the old nerve trunk. As the limb segments are formed, this cord of 
cells is joined by similar strands from the various segments. Within 
these cords the axis cylinders of the nerve fibers develop, and from the 
cells composing the cord the sheath cells are differentiated. The 
important question is as to the origin of these cords of cells; have 
they migrated outward from the sheath cells of the old nerve trunk, or 
are they derived from the ectodermal cells of the regenerating bud ? 
The present observations support the latter conclusion. Among 
the old sheath cells no evidence of cell division was obtained, 
nor was there observed any extensive proliferation or outward migra- 
tion of the sheath cells from the old nerve trunk. On the other 
hand, cells from the first-formed ectodermal plate migrate inward 
toward the injured nerve, as has already been described in the early 
