Tissues in the Crustacean Limb. 137 
only noticeable difference being that the epidermal cytoplasm next to 
the chitin takes a slightly darker stain (Fig. 16). The question 
now arises, what is the relation of the developing myofibrille to the 
external epidermal cells and how is their attachment to the chitin 
established ¢ 
In studying the regenerating tissue at the stage when the myo- 
fibrille can just be identified (Fig. 16, six days, six hours), an im- 
portant fact becomes evident, viz., that fibrils develop in the epider- 
mal layer of cells, simultaneously with the differentiation of the 
myofibrille in the internal cells, apparently without any discon- 
tinuity between them. In other words in early differentiation the 
myofibrilla can be traced into the layer of epidermal cells, and in 
some cases at even this early stage (six days, six hours) myofibrils 
were found continuous even through to the chitin. 
The differentiating muscle fibrils early assume a characteristic 
striation (Fig. 18, mf). At the same time more clearly defined 
differences are becoming evident between the epidermal and muscle 
cells. The muscle nuclei are more nearly spherical in form, while 
the epidermal cells take a distinctly heavier stain. Small vacuoles can 
also be observed at the boundary between the two groups of cells. 
The striation of the muscle fibrils gradually becomes more distinct. 
These striations could be traced beneath the inner surface of the 
epidermis, frequently extending to the level of the proximal ends of 
the nuclei (Fig. 18). No evidence was obtained, however, that the 
fibrillee are ever striated entirely through the epidermis to the chitin. 
A definite boundary (basement membrane) now becomes apparent 
between the epidermal and muscle cells (Fig. 19, em). There is an 
interesting variation in the general level of the boundary or inner 
surface of the epidermis in the region of muscle attachment. The 
epidermis between the attachment of two or more muscles, frequently 
projects inward in rounded elevations or broad papilla. The result 
is that in the immediate vicinity of the muscle attachment the inner 
surface of the epidermis appears to follow the fiber for some distance 
toward the chitin, thus forming a sort of pit, through the bottom of 
which passes the muscle fiber (Figs. 19 and 20). Since this eleva- 
tion of the epidermis increases in the later stages of the regenerating 
