MUSCLE ATTACHMENTS AND EPIDERM IN CRAYFISH 391 



which project down into the muscle are derived from the network 

 which is produced by anastomosis of tendon fibrils at the base of 

 the tendon. Some of the fibrils pass down into the muscle for a 

 considerable distance. The •' splicing ' or 'dove-tailing' of the 

 muscle fibrils and tendon fibrils is seen very clearly in this figure 

 and in figure 1. 



The findings of previous workers in regard to the union of tendon 

 and muscle in Arthropods have been presented in the first part 

 of this paper and it is unnecessary to restate them here. Riley 

 seems to be the only author who has observed anything like 

 the process described here. According to Riley, " . . . . 

 there occurs a slicing or fusion of the two types of fibrils, the base- 

 ment membrane being lacking at the point of contact." If the 

 network or layer of fibers at the base of the tendon and in the lower 

 portion of the hypodermis is to be interpreted as a basement 

 membrane, then Riley's statement in regard to this structure is 

 not correct for the crayfish, as is seen by an examination of figures 

 1 and 5. However, the writer can see no reason for calling this 

 structure a basement membrane. The variations to which it is 

 subject are shown in the five figures presented here, and it hardly 

 seems reasonable to call such an indefinite structure a basement 

 membrane. 



For the crayfish it seems almost certain that the tendon fibers 

 are within the hypodermal cells and not between them, as is 

 claimed by Frenzel, Nicolas, Ide, Pantel and Hecht. The fact 

 that the tendon fibers seem to be bundles of ordinary supporting 

 fibrils is in favor of the former view, and figures 1, 3 and 5 can be 

 interpreted in no other light. The fibers occupy the greater part 

 of the cell, but a small quantity of cytoplasm can always be seen 

 between them, and the original hypodermal nuclei are still to be 

 seen. The final solution of this question will depend on ontogene- 

 tic studies, but for the present the view presented here seems to 

 be the most reasonable one. 



The origin of the muscles in the Arthropoda is still being debated, 

 some authors claiming that they are of mesodermal origin, others 

 that they are derived from ectoderm. It is very likely that they 

 may be derived from both sources, which is probably also true 



