Tissues in the Crustacean Limb. 123 
more lightly stained area is now at the upper and left side, whereas 
the darker zone at the proximal end is directed toward the right side 
of the nucleus. In studying the sections, a considerable enlargement 
or expansion of the nucleus is frequently found at the distal end, or 
region of the light zone, in some cases being even more marked than 
is shown in nucleus d (Fig. 13). At this stage of migration the 
former karyosomes have now almost disappeared. The chroinatin 
has disintegrated into fine granules, which appear to be either per- 
ipherally distributed or axially segregated into a dense mass. It is 
also to be observed that the denser central mass is always located at 
the darker pole of the nucleus. 
The significance of this polarization of the nuclear contents (if it 
may be thus designated) is not readily apparent. It can hardly be 
considered an artifact resulting from faulty fixation or staining. 
On the contrary, its regular occurrence, its reversal at opposite sides 
of the limb, the frequent expansion of the lighter pole, the associa- 
tion of the axial chromatin mass with the darker pole, together with 
the fact that similar conditions are even more conspicuous in later 
stages of the regenerating limb as will be preseatly described, is 
evidence which supports the conclusion that this polarization of 
nuclear contents is a characteristic structural feature of certain phases 
in the regenerative activity of the epidermal cells.? 
Rand’s (’04) discussion of the regenerating epidermis of the 
earthworm applies equally well to the migrating epidermal cells in 
the regenerating lobster’s limb. He says: “The movement of the 
cells at the margin of the layer is not a passive one, resulting from 
external pressure. Nor is there the slightest ground for supposing 
the existence of a force acting from a position anterior to the cut 
epidermal edges and serving to pull the epidermal cells over the cica- 
trix. We are compelled, therefore, to look in the individual epi- 
dermal cell itself for the immediate source of the activity. This 
activity, by whatever mechanism effected, must be occasioned by an 
agency external to the cell, viz., by some factor of the conditions 
resulting from the injury.” Beyond this we must admit that as yet we 
“The resemblance of the “polarized” nuclei in appearance to certain stages 
of synapsis of the nuclei in spermatogonia may be noted, although it has 
suggested no further explanation of the “polarization.” 
