Tissues in the Crustacean Limb. 145 
Let us consider first the broad sheets of tissue extending from one 
wall to the other within the limb segment. As the segments of the 
regenerating limb become well differentiated, there occurs an exten- 
sive inward migration of ectodermal cells in those regions where there 
later develop the bands of connective tissue characteristic of the adult 
limb. This migration is especially well defined in the regenerating 
meropodite. The epidermal cells in this region migrate inward from 
the epidermal wall and form a broad sheet extending across the central 
cavity of the segment (Fig. 26, 7c). The nuclei become elongated, 
and distinct fibrils arise in the cytoplasm. At later stages (Fig. 27, 
eleven days, ten hours) the cytoplasm, with the exception of the 
epidermal cells next the exoskeleton, becomes vacuolated. These inter- 
cellular spaces then become filled with blood plasm (Fig. 27, 5), 
and eventually there is thus formed the vascular and connective 
tissue characteristic of the fully developed limb. 
It seems evident, therefore, that in at least certain regions of 
the regenerating limb a supportive and apparently true connective 
tissue may be derived from ectodermal cells. Nor is this divergent 
from conditions found in normal development among crustacea, for 
Claus (’86) in his work on the embryology of Branchipus, concludes 
that the broad connective tissue bands which unite the opposite sur- 
faces of the integument, “sind Erzeugnisse der Chitinogenzellen der 
Hypodermis”’, and quotes Braun to the effect that in the crayfish 
it is “nicht méglich, eine scharfe Grenze zwischen den Erzeugnissen 
von Chitinogenzellen und den mesodermalen Bindegewebebildungen 
festzustellen” (pp. 22-23). 
We may now briefly consider the tissue characteristics of the so- 
called “Grenzlamelle.”” In the description of the regenerating mus- 
cle, reference has been made to the fact that a “Grenzlamelle” is 
not present under the epidermis at the time when myofibrille begin 
to differentiate. It is only after the muscle fibers and the epidermis 
have become well developed that an epithelial layer is formed over 
the inner surface of the epidermal cells. In various parts of the 
limb this “Grenzlamelle” is apparently the only tissue between the 
blood sinuses and the epidermis, and consequently in at least certain 
regions it seems to serve as a vascular epithelium (cf. Williams, ’07, 
p. 155). 
