Tissues in the Crustacean Limb. 133 
(mf) taking a darker stain with Congo red. The very earliest fibrils 
are, however, not so readily identified on account of their similarity 
to the cytoplasmic network and their close relations with it; for at 
the time of their first differeniation the fibrille stain but slightly with 
Congo red, are irregular or wavy in structure, and appear to be in 
continuity with the cyto-reticulum. These characteristics indicate 
that the contractile elements in the regenerating limb of the lobster 
may be derived trom the cyto-reticulum, and consequently favor the 
“network” rather than the “fibrille” theory for the origin of striated 
muscle fibers. It should be added, however, that no such constancy 
in relation was observed between the cytoplasmic network and the 
fibrille, as would seem to be necessitated by MacCallum’s (’98) 
theory for striated muscle of vertebrates. 
As late as the seventh day of regeneration each myofibril (Fig. 23) 
still retains its individuality as a single structure. During the eighth 
day, the fibrils began to appear double or in pairs. In Fig. 24 one 
of the fibrils is still a single structure, but the remaining eight fibrils 
are in pairs, the members of each pair appearing in cross section as 
half cylinders. At later stages of differentiation each pair of 
fibrillee becomes represented by a group of four fibrils; the number 
in each group then increases until as many as twenty fibrille could 
be counted in each bundle, which in transverse section may now be 
recognized as a “Cohnheim’s area”. As to how the fibrillee multiply, 
the evidence from appearances in both cross and longitudinal sections 
of the regenerating muscle in this invertebrate supports the conclu- 
sions of Heidenhain, Eycleshymer, and other investigators for the 
striated muscle of vertebrates,—that an increase in the number of 
myofibrillz arises through longitudinal division of the fibril. 
Preceding the first division or splitting there is a marked increase 
in the diameter of each fibril (compare Figs. 22 and 23). At the 
same time the cytoplasm immediately surrounding the fibril becomes 
less granular and is distinguished from the neighboring cytoplasm 
by its lighter stain (Figs. 23 and 24). The developing fibril is 
consequently enclosed by a sheath of modified cytoplasm, which is 
evidently correctly interpreted as representing the beginning of a 
Cohnheim area of the mature muscle fiber. 
