Tissues in the Crustacean Limb. 153 
a. A muscle crossing the joint, but degenerating during the fifth 
larval stage. 
b. Valves in the venous blood sinus which prevent excessive hem- 
orrhage after the autotomy of the limb. The valves persist through- 
out life. 
The following conclusions concern the regeneration of the limb 
after autotomy. 
2. The first layer of cells, 
wound is formed by the migration of epidermal cells. 
aside from the blood clot, to cover the 
3. The nuclei of these migrating cells enlarge and become some- 
what wedge-shaped. The karyosomes disintegrate into fine granules 
which tend to collect at the proximal end of the nucleus. Thus the 
nuclear contents appear polarized into an inner zone of darkly 
stained chromatin, and an outer zone relatively free from chromatin 
granules. 
4. The significance of this polarization is not clear, but its regular 
occurrence, and is reversed on opposite sides of the limb, together 
with the frequent expansion of the higher pole, and the association 
of axial chromation masses with the darker pole, indicate that it is a 
phenomenon characteristic of certain stages in the regenerative activ- 
ity of the epidermal cell. 
5. During these nuclear activities the cytoplasm loses its suppor- 
tive fibrille and reticular structure, and becomes finely granular. 
Although definite cell membranes are never evident, there are more 
or less clearly defined cytoplasmic units around each nucleus.> 
6. Mitotic cell division begins after the formation of the first 
epidermal plate over the wound. The formation of this plate seems 
sufficiently accounted for by the volumetric increase of the cytoplasm, 
together with the enlargement and wider separation of the nuclei 
of the migrating cells. 
7. The wall of the regenerating bud, and a large part, if not the 
entire core of cells filling its interior, are derived from epidermal 
cells. Since there is no conclusive evidence of either cell division 
or migration among the old muscle and connective tissue cells, these 
tissues appear to contribute little, if anything, to the formation of the 
new limb bud. 
