134 Victor E. Emmel. 
Reference has just been made to the fact that each myofibril 
retains its individuality as a single structure until the seventh day of 
regeneration. At this time the light and dark bands of the myofi- 
brill: are already present, but the ‘‘Z” line, or membrane of Krause, 
could not be observed. By the eighth day the ‘‘Z” line had become 
differentiated, the first indication of these lines being found during the 
early part of the seventh day. It appears, therefore, that the “Z” 
lines differentiate later than the light and dark bands. In this 
respect the differentiation of the regenerating crustacean muscle 
resembles the histogenesis of the striated muscle as found in the 
21 mm. pig embryo, where, as described by Bardeen (700, p. 392), 
“The fibril bundles are composed of longitudinal fibrils made up 
of longer deeply staining segments alternating with shorter lighter 
segments which do not stain. It is only in older embryos that lines 
through the light areas, corresponding to Krause’s membrane, may 
be clearly distinguished.” 
The problem of the origin of the multinucleated muscle cell and its 
sarcolemma in the regenerating tissue of the lobster differs from the 
same problem in the vertebrates, because in the former there appar- 
ently are no myoblasts concerned in muscle formation. 
The structure in the adult muscle of the lobster, which is known 
as the sarcolemma, is a membrane surrounding the muscle fiber and 
containing elongated nuclei. This membrane is usually regarded 
as developed ontogenetically from connective tissue elements. No 
evidence, however, was obtained for a similar origin during regenera- 
tion. When the myofibrille first appear in regeneration, definite 
cell membranes cannot be identified, but the sarcolemma first appears 
in a later stage of differentiation (Fig. 25, sa), and seems to arise 
by a modification of the cytoplasm of the ectodermal cells. 
During the differentiation of the myofibrille (which are at first 
placed centrally and later become located eccentrically in the muscle 
fiber), the nuclei undergo certain characteristic changes in form and 
structure. The elongation of the ectodermal nuclei, as they pro- 
liferate and migrate inward at the joint, has already been described. 
As the migration advances, the nuclei in the region where the myo- 
fibrils differentiate, assume a peripheral position in the developing 
