130 Victor E. Emmel. 
compared with the more expanded outer peripheral region. Here 
again the significance of this apparent polarization of the nuclear 
contents is not clear, but, as was mentioned in the ease of migrating 
nuclei, the general occurrence of these structural characters in dif- 
ferent specimens and with different stains, and the variation of the 
polarization of the elements in conformity with variations in the 
position of the nuclei, hardly permits of the interpretation of this 
phenomenon as a mere artifact. 
As the invagination of the ectodermal cells at the joints advances, 
the involved nuclei undergo further typical changes. Fig. 17 (seven 
days, six hours) represents several nuclei taken at a later stage in 
the development of the same invagination shown in Fig. 15 (five days, 
ten hours). A marked difference is at once apparent in these two 
groups of nuclei. In Fig. 17 the chromatin is evenly distributed, 
and the nuclei have become elongated to such a degree that frequently 
they are more than two and one-half times longer than at an earlier 
stage. See Fig. 16 (six days, six hours) for intermediate stages. 
It will also be observed in this latter figure that at the center of the 
invaginating mass of cells, a thin lamella of chitin (mp) is now be- 
coming evident, which later serves for muscle attachment. 
VI. Origin or New CBrEtts. 
We have already learned that the regenerative process is initiated 
by epidermal cells, which migrate across the wound, form a layer 
over the injured surface, then multiply by mitotic division and evag- 
inate to form the new bud. It is evident, therefore, that the outer 
layer of cells covering the regenerating limb bud is entirely of ecto- 
dermal origin. 
The evagination of the ectodermal cells encloses a cavity at the 
center of the bud, which becomes filled with a core of cells. The 
origin of this core of cells becomes at once a vital question in the 
further study of the histogenesis. Since the interior of the fully 
developed limb consists of such tissues as striated muscle and con- 
nective tissue, it might be expected that this internal core of cells 
would be derived from similar tissues in the old stump, and conse- 
quently would be mesodermal in origin. 
