326 PATTERNS AND PROBLEMS OF DEVELOPMENT 



constitution of organs, emerge as minute individuals. A considerable de- 

 gree of control of fragmentation is possible. Animals kept at low tem- 

 peratures increase in length slowly and do not fragment, even when they 

 have attained much greater length than the fragmentation length at 

 higher temperatures. Occurrence and course of degeneration and fragmen- 

 tation are also influenced by character of food supplied." It is not certain 

 in this case whether physiological isolation initiates the degenerative 

 changes at the posterior end or whether the degeneration results in physio- 

 logical isolation, but the latter appears more probable. It is evident to 

 observation that the posterior region about to separate as fragment has 

 ceased to behave like a normal posterior end, shows little or no reaction to 

 stimuli, and approaches spherical form. 



Autotomy or fragmentation results from irritation in various nemer- 

 teans and some annelids (e.g., Lumhriculus) , apparently in consequence 

 of independent muscular contractions in different regions. Excitation of 

 postcephalic regions evidently isolates them physiologically for the mo- 

 ment, and independent contraction and physical isolation follow. A local 

 dominance of an excited region may perhaps be a factor in determining 

 length of pieces, that is, excitation at one level may prevent independent 

 reaction within a certain distance, which will probably differ with body- 

 level and other conditions. 



REDUPLICATION OF PARTS 



An actively growing part — for example, a tentacle — is apparently able 

 to inhibit development of another tentacle within a certain distance of 

 itself; but, if two or more such parts develop simultaneously, they appear 

 to have Httle or no inhibiting effect on each other. These relations appear 

 very clearly in the scyphozoan Haliclystus. After removal of the margin 

 of the umbrella by transverse section the eight tentacle groups and mar- 

 ginal organs appear simultaneously and develop rapidly, but after ob- 

 lique section reconstitution at the more distal levels retards or completely 

 inhibits that at more proximal levels (pp. 50-52 and Fig. 24). A similar 

 dominance of distal over more proximal levels appears in the scyphistoma 

 of Amelia, though somewhat less effective. Tentacle development is 

 greatly retarded at the more proximal levels of oblique section; but new 

 tentacles appear at these levels and develop slowly, the normal number, 

 sixteen, being gradually approached and perhaps attained finally (Child). 



The activation at a certain stage of development of the segment-forming 



" Child, 1913c, 1914^; W. A. Castle, 1928. 



