Tissues in the Crustacean Limb. 147 
325). Haseman (’07) concluded that in the regenerating cheliped 
of the crayfish, differentiation proceeded in a disto-proximal direc- 
tion. Haseman, appreciating the importance of determining whether 
changes in external form could be safely taken as an index of internal 
changes, gave some attention to the internal differentiation of tissues. 
But as a rule conclusions upon this subject have been largely based 
upon evidence derived from modifications in external structure, 
rather than from internal cytological and histological changes in the 
segment tissues. In view of this fact, the following study has been 
made with the purpose of obtaining further data upon the direction of 
differentiation among the internal regenerating tissues. 
The results to be described were derived from the study of serial 
sections of somewhat over a hundred regenerating chele at various 
stages of development. In tracing the progress of differentiation 
it becomes necessary to rely on such criteria as are most clearly 
defined. In the present case the following structural characters 
have been chosen on account of their ready identification by means 
of selective stains: (1) the formation of chitinogenous muscle plates 
or tendons; (2) the differentiation of myofibrille; and (8) the 
striation of the muscle fibrils. 
In the description of the differentiation of the regenerating limb, 
it will be recalled that distal to the breaking joint there are five 
segments. Kach segment contains a pair of muscles controlling the 
extensor and flexor movements of the next distal segment (see Fig. 
37). It is important to note that in each pair of muscles the flexor is 
the larger, and that of the four pairs of muscles, it is the first and third 
pairs (counting from the distal end) which contain the largest muscles, 
7. €., the muscles which lie in the propodite and meropodite. Each 
muscle takes its origin from the exoskeletal wall of the segment in 
which it lies, and is inserted upon the chitinogenous plate extending 
proximally from the next distal segment. The differentiation of these 
chitinogenous plates will first be considered. 
1. The Chitinogenous Plates or Muscle Tendons.—The differen- 
tiation of chitin within the joint invagination is indicated by a char- 
acteristic brick-red stain reaction with Congo red, or bright blue with 
Mallory’s connective tissue stain. ‘The developing chitin is thus 
