Tissues in the Crustacean Limb. 125 
In regard to the segmentation of the ovum, Professor Minot (’08) 
emphasizes this increase in nuclear material as a characteristic 
feature of “the process of rejuvenation,’ and as a result of his 
studies of the structural relations of cytoplasmic and nuclear mate- 
rial during the development of the organism, concludes “that as we 
define senescence as an increase and differentiation of the proto- 
plasm, so we must define rejuvenation as an increase of the nuclear 
material,” p. 167. Whether we shall be justified in attaching a 
similar significance to the corresponding nuclear changes in regen- 
elation, or whether in the latter case the nuclear phenomena are 
fundamentally different in nature, are questions for further inves- 
tigation. 
4. Cytoplasm.—In describing the cytoplasm of the epidermal cells, 
it is necessary to consider the cells collectively, on account of the ill- 
defined or even non-existent cell boundaries. In the normal or rest- 
ing cell the cytoplasm has a reticular structure of rather even appear- 
ance and is traversed by larger delicate supportive fibrils lying more 
or less parallel to the long axis of the cell. As the epidermal cells 
migrate over the surface of the stump, their cytoplasm ceases to be 
characterized by the presence of fibrillar structures, but appears to 
be composed of fine granules rather uniformly distributed (Figs. & 
and 9). A definite reticular structure is no longer evident. The 
inner surface of this sheet of migrating cells, instead of being smooth, 
presents an uneven contour due to numerous rounded inward projec- 
tions of cytoplasm. These projecting masses of cytoplasm never 
show definite limiting membranes, but their boundaries can, however, 
be traced upward into the general layer of cytoplasm as far as the 
level of the epidermal nuclei, the relations of each mass being such 
as to indicate that it represents the unit of cytoplasm coming under 
the influence of each nucleus. The volume of cytoplasm appears 
to have increased, as is indicated by the wider separation of the 
nuclei and the greater depth of the epithelium after its migration 
has been completed. 
One result of these observations on the epidermal cells is to empha- 
size the fact that in the migration of these cells as the first step in 
regeneration, there occurs a parallel series of definite structural 
