782 MORPHOGENESIS 



Differences in the degree of reorganization in anterior and posterior 

 halves of Spathid'mm spathula are clearly described by Moore (1924). 

 In this species the alterations prior to restoration do not follow the same 

 course, but vary with the size of the fragment and with the condition of 

 the organism at the time of cutting. She observed no instance in which 

 the cytostome remained entirely unchanged throughout the course of 

 regeneration in anterior halves. Shortly after the cut surface heals, an- 

 terior pieces begin to round out at the apex. The assumption of spherical 

 form extends to the oral region, the oral parts become less distinct, the 

 neck disappears completely, and finally only an indication of the oral 

 lips is apparent. Occasionally dedifferentiation proceeds further, the frag- 

 ment forming a complete sphere with all traces of the oral apparatus 

 totally obliterated before redifferentiation sets in. The posterior halves 

 require a longer time for the restoration of form, although they contain 

 none of the original oral parts to be resorbed and remodeled. 



Heterotrichs are not strikingly different from holotrichs in regard to 

 the extent of dedifferentiation following injury. The greater the relative 

 size of the regenerator in such forms as Spirostomum (Sokoloff, 1923), 

 the greater its regenerative capacity, and the sooner are such parts re- 

 stored to complete, full-size individuals. According to Moore (1924), 

 the oral structures in anterior pieces of Blepharhma do not dedifferenti- 

 ate unless they are injured when the cell is operated upon. A somewhat 

 similar condition obtains in Bursaria (Lund, 1917). In this heterotrich 

 the Anlage of the gullet in reorganizing individuals may be torn slightly 

 without provoking complete dedifferentiation. The injured region per- 

 sists as an abnormal part of the gullet, which is normal in all other 

 respects. This appears to be a clear-cut demonstration of embryonic 

 localization as found in metazoan embryogeny, a definite spatial corre- 

 spondence between the undifferentiated and the fully formed parts. In 

 Bursaria, however, there is considerable variation in the degree of injury 

 required to produce more drastic regenerative measures. If sufficiently 

 great, an injury to the gullet rudiment causes complete dedifferentiation, 

 such that all visible traces of the rudimentary structure disappear before 

 redifferentiation begins. The regenerative processes in hypotrichs tend 

 to obey an all-or-none rule for, once initiated by a requisite degree of 

 injury, the dedifferentiation proceeds to completion. It is noteworthy, 

 moreover, that minimal injuries to certain of the motor organelles will 



