Tissues in the Crustacean Limb. 149 
The first point becomes especially interesting when it is considered 
that according to the requirements of the theory that differentiation 
proceeds in a disto-proximal direction, the next chitin muscle plate 
to develop after those of the propodite should have been in the third 
segment or carpopodite instead of the fourth segment or meropodite. 
Serial sections of the carpopodite were carefully examined, but there 
was no evidence that any chitinization had yet taken place for the 
tendon plates in this segment. 
c. Hour Days and Twenty-two Hours (Fig. 33).—At this stage a 
fourth chitinous tendon (77) has become evident. This tendon is 
in the third segment and is the one involved in the development of 
the tlexor muscle. The extensor (e') and flexor (r!) in the propo- 
dite, and the flexor (7*) in the meropodite, show an advance in differ- 
entiation as compared with the preceding stage, but otherwise there 
is no evidence of chitin differentiation \in any of the remaining 
muscles. Consequently at this stage only four chitinogenous plates 
or tendons are present; two in the propodite and one in each of the 
next two proximal segments. The important point is that in each 
case it is the tendon for the flexor muscle which differentiates first 
in each segment. 
d. Five Days and Six Hours (Fig. 34).—The tendon for a second 
extensor muscle (¢*) is now beginning to differentiate. Here again 
the next tendon to develop is not for the extensor in the carpopodite, 
but for the one in the meropodite. At this time two other chitinous 
tendons (e*) are also just becoming evident in the fifth segment or 
ischiopodite. The muscles correlated with these last two tendons, it 
will be recalled, are both extensors. 
e. Live Days and Nine Hours (Fig. 35).—The chitinous tendon 
(e*) for the extensor of the carpopodite is now present. This intro- 
duces, therefore, the last of the eight muscle tendons to differentiate. 
f. Len Days and Two Hours (ig. 36).—By this time the eight 
chitinogenous tendons and muscles of the regenerating limb have 
become fully developed. The animal is now about to undergo the 
process of moulting, in which the regenerated limb will be liberated 
from its membranous sac and become a functional appendage. 
In summarizing these observations it may be stated that the chitin- 
