Tissues in the Crustacean Limb. 143 
stages of the regenerating bud (Fig. 9). At later stages this inward 
migration of ectodermal cells is quite marked in certain regions 
(Fig. 28, n), where they evidently contribute to the formation of 
the cords of cells just described. The distal end of each cord 
always contains a larger number of cells than its more proximal end, 
giving it a club shape, with the smaller end joined to the old nerve. 
Occasionally mitotic figures were found, but they always occurred 
among the more distal eclls of the cords. The nuclei of these distal 
cells were generally large and spherical in form, but in passing proxi- 
mally toward the old nerve trunk, there was a gradual transition 
from the spherical nuclei to the long flat nuclei of the sheath cells, 
indicating a direct differentiation of the neurilemma from these ecto- 
dermal cells. 
These observations on the lobster, together with the results of 
Reed’s (’04) on the crayfish, and Steele’s (’07) on the nerve endings 
in the ommatidia of the eye, furnish collective evidence for the par- 
ticipation of ectodermal cells in the regeneration of the nerve fibers 
in crustacea. It remains to be determined, however, whether this 
conclusion can be extended to arthropods in general. For it must 
not be overlooked that different results have been obtained by Ost 
(706) in his study of the regenerating antenna of Oniscus, in which 
form he finds that the neurilemma regenerates ‘‘durch Nachschieben 
vom proximalen Ende her . . . Die Regeneration des Antennen- 
nerven von Oniscus geht also nach meinen Beobachtungen durch 
direktes Auswachsen junger Nervenfasern aus dem alten Stumpf vor 
sich” (p. 313). It is noteworthy that Ost was unable to obtain 
any evidence of cell division among the sheath cells, for he states, 
“Mitosen oder sonstige Teilungsvorginge konnte ich an diesen Ker- 
nen freilich nie beobachten,”’—a result corresponding with the con- 
ditions found in the regenerating lobster’s limb, 7. e., in the region of 
the old nerve trunk. 
By the fifth day of regeneration (five days, ten hours) the differ- 
entiation of an axis cylinder within the cord of cells just described, 
had advanced sufficiently to present delicate neurofibrille.® These 
‘The nerve fiber thus appears to develop earlier than the muscle, for in 
all cases regenerating nerve fibers were found before myofibrille could be 
detected. 
