234 Mary Isahelle Steele 



incomplete. This fact suggests that the regeneration was not 

 influenced bv a fixed set of internal conditions. In the usual 

 cases of regeneration and embryonic development, whatever the 

 determining factor or factors may be, it is recognized that we may 

 expect certain structures to appear in connection with a given set 

 of external and internal conditions. 



In the development of this heteromorphic appendage, however, 

 conditions seem more variable. As a consequence it shows con- 

 siderable variety of form. In some cases the appendage is but 

 little more than a slender horn-like projection, in other cases the 

 appendage may be curved inward toward the median line, project 

 forward at the angle of the eye or curve backward until the free end 

 touches the margin of the head. (Compare Figs. 23, 38 and 41.) 

 Again from the very first moult the appendage may appear as a 

 single flagellum-like structure or as a pair. None of the hermit 

 crabs, however, have regenerated a heteromorphic appendage 

 composed of two flagellum-like parts. But in my previous 

 observations upon crayfish [loc. cit.) two or three instances were 

 noted in which the appendage appeared double at the time of the 

 first moult. Herbst has also noted what he regards as an endo- 

 podite and exopbdite in several instances. The appearance of the 

 single structure in some cases and the double one in some others 

 can perhaps be explained by the supposition that the nerve fibers 

 become separated into two masses in some instances and remain 

 as a single trunk in others. Miss Reed {loc. cit.) found that when 

 the stump of the leg of a crayfish or hermit crab was split longitu- 

 dinally in some instances two legs were regenerated from a single 

 stump and in other cases only one. Sections of such legs showed 

 that the end of the nerve stump had been split in the cases in 

 which two legs regenerated and that the nerve stump had not been 

 split when only one leg was regenerated. A similar result might 

 follow in the development of the heteromorphic appendage if the 

 nerve trunk became separated into two bundles by the interpo- 

 sition of another sort of tissue. 



An explanation of the antenna-like form of the heteromorphic 

 appendage having been suggested, attention should now be 

 directed toward an explanation of its inner structure, which is also 



