236 Mary Isahelle Steele 



somatic appendage. The nerve trunk of an appendage is asso- 

 ciated with ganghon cells only at its central end, not with ganglion 

 cells at its peripheral end, as distinguished from the optic nerve in 

 its relation to the optic ganglion, consequently in removing an 

 appendage no parts have been removed that would not be likely to 

 again regenerate in a similar manner. While in animals as highly 

 specialized as the hermit crabs we do not find the ganglion parts of 

 the nervous system regenerating. 



For the negative cases that appear after the entire optic ganglion 

 has been removed, it is evident that no real explanation can be 

 offered until a more adequate understanding of the process of 

 growth and development has been reached. Although we may 

 fully recognize the fact that great differences exist in the physiolog- 

 ical activity of the various individuals and that the external condi- 

 tions are subject to numerous variations, these facts alone will not 

 account for the great number of negative cases which result. In 

 addition to these it seems necessary to recognize an individual 

 variation in the quality of the tissues. Nothing short of some 

 specific inherent individual difference seems sufficient account for 

 the fact that only an occasional hermit crab regenerates a hetero- 

 morphic appendage. The ability to regenerate a heteromorphic 

 appendage in place of an eye which appears as an individual 

 variation in hermit crabs and Crangon and other genera seems to 

 be entirely wanting in at least one species of Palaemonetes. Or if 

 not entirely wanting it appears so rarely that even after a great 

 number of experiments and observations it is apparently absent. 



In summing up the foregoing discussion it is apparent that a 

 weight of responsibility has been placed upon the nervous system. 

 Numerous observations, however, have left no doubt that the 

 nervous system does exercise an important physiological influence 

 upon the other tissues of the body, both in ordinary growth 

 phenomena and in regeneration. Child ('04) observed in oper- 

 ating upon Leptoplana that if more than half of the cerebral 

 ganglion was removed a new head did not regenerate. This was 

 true regardless of the plane in which the cut was made, a fact 

 which seems to indicate that the mass of nervous material is an 

 important factor in the case of Leptoplana at least. Wilson ('03) 



