Tissues in the Crustacean Limb. 129 
characteristic structural changes. The mitotic figures occur at the 
periphery of the bud, the spindle being almost invariably parallel 
with the surface. A dividing nucleus is rarely found at a deeper 
level. The cytoplasm is granular in appearance. No cell walls were 
detected. In the vicinity of the mitotic figure, however, the mass of 
cytoplasm immediately surrounding the dividing nucleus is less 
granular and takes on a lighter stain. 
Fig. 15 (five days and ten hours after operation) represents a 
group of epidermal cells in the region of the invagination for the 
flexor muscle of the fourth segment. The four nuclei at the right 
in this figure are migrating inward in the process of invagination. 
The three nuclei at the left are in the epidermal wall of the segment, 
while the remaining nucleus, in the prophase (?) of mitosis, occupies 
the typical superficial position. In these nuclei there may again be 
seen a polarization of the nuclear material similar to that described 
in the earlier stages of the regenerating cells. The dark staining 
chromatic elements are found at the inner or more mesial pole of the 
nucleus (with the exception of the nucleus undergoing mitosis). At 
the same time the opposite pole of the nucleus takes only a lght 
stain and evidently contains a much smaller amount of chromatin. 
Here again the position of the plane of division between these light 
and dark zones varies with the position of the nucleus. (In Fig. 15 
the left and right parts are respectively distal and proximal with 
reference to the limb.) The conditions odnerally found may be 
stated as follows: In nuclei lying approximately transverse to the 
long axis of the regenerating bud, the darker staining material is at 
the inner end or pole of the nucleus. In nuclei not lying in a trans- 
verse plane, as occurs in the region of joint formation, the chromatic 
material, although still found in the inner region of the nucleus, 
now becomes shifted either to the proximal or to the distal side of 
the nucleus according as the inner pole of the nucleus is directed in 
a distal or a proximal direction. The darker pole also frequently 
contains a darker body or nucleolus, while at the lighter pole of the 
nucleus there was usually found a reticular arrangement of fine 
granular elements. The inner or darker pole of the nucleus was 
generally found smaller and more tapering or pointed in form as 
