20l] REGENERATION OF CRAYFISH APPENDAGES 1 3 



external. If the crayfish is old and the break does not occur at 

 or a short time after a moult, the new appendage is not likely to 

 appear until after the next moult. In a young crayfish the new 

 limb may appear whether there has been a recent moult or not. 

 As has been described by Herrick ('95, p. 164) for the lob- 

 ster, the first external indication of a new appendage is the ap- 

 pearance of a small white papilla on the end of the old stump. 

 The bud is at first semi-transparent white, and, as it grows, con- 

 strictions appear which indicate the future joints. There are at 

 first three transverse constrictions, and then on the most distally 

 constricted portion there appears a longitudinal fissure which 

 marks the separation into dactylopodite and propodite. Figs. 

 1-4, PI. I, show the appearance of the papilla and its subsequent 

 transformations up to the time of a moult. Fig. i shows the 

 stump before the papilla appears; Fig. 2 its first appearance; 

 Fig. 3 the rudiments of four new segments formed, and in Fig. 

 4 all five new segments are constricted oflf. After the papilla 

 once appears, if conditions are favorable, the subsequent growth 

 is rather rapid, at least until the confining cuticle has reached the 

 limit of its extensibility. As the limb-rudiment grows it 

 changes color from a somewhat opaque white to a rather trans- 

 lucent white, then to pink, and finally to bright red. Usually all 

 of the constrictions which mark the future segments of the ap- 

 pendages have appeared before the regenerated portion turns 

 red. The cuticle which covers the limb-rudiment soon reaches 

 its limit of extensibility, and no further external increase in 

 size is apparent until after the next moult. The regenerated 

 appendage does not then exceed half an inch in length, but in- 

 side the confining cuticle a very much greater growth has taken 

 place, as is manifest as soon as a moult occurs. How great a 

 growth may be compressed within the cuticle is shown by a 

 comparison of Figs. 5 and 6, PI. II. Fig. 5 shows the old shell 



