798 MORPHOGENESIS 



dissimilar, it would be of interest to discover the conditions under which 

 differentiation leads only to restoration or to division. 



Dembowska (1926) made an interesting discovery while working 

 with Uronychia. Four hours after division, two sister cells were operated 

 on in different regions. The individual cut as shown in Fig. 182 A di- 

 vided five hours later, giving rise to two normal individuals. After the 

 same interval of time the individual cut anteriorly (Fig. 182 B) began 

 to regenerate in the usual fashion; it produced a single new individual. 

 Dembowska suggested that the type of operation determines the mode 

 of reorganization. 



A similar condition obtained in M. E. Reynold's (1932) experi- 

 ments on Oxytricha jallax. Amicronucleate individuals were able to 

 regenerate when cut late in the interphase, from approximately five 

 hours after one division until the beginning of the next. The majority 

 of the variously operated organisms reorganized by division, rather than 

 by the ordinary mode of restoration. In this species injury hastened the 

 divisional process. Reynolds thinks that once the division processes are 

 under way, they are completed in spite of moderate surgical disturb- 

 ances. 



Injuries sustained shortly before the onset of division are not repaired 

 by divisional reorganization in some species. In 'Paramecium caudatum 

 injured cells need not regenerate to divide, and may or may not re- 

 generate before dividing (Calkins, 1911b). A truncated Paramecium 

 frequently gives rise to a truncated and a perfectly formed daughter. The 

 former occasionally divides again before regenerating (Fig. 183). 



It is also true that regeneration may suppress or supplant division in 

 certain species. Hartmann (1924), for example, maintained Amoeba 

 polypoda in an undivided state for long periods by repeatedly cutting 

 away portions of the cytoplasm. Each time that growth reached the point 

 of an impending division, he removed cytoplasmic fragments amounting 

 to as much as one-third of the total cell volume. In one experiment A. 

 polypoda was operated on 21 times within 25 days. In this manner divi- 

 sion was held in abeyance while the control strains divided 11 times. 

 Another experiment continued for 42 days, during which the operated 

 individual regenerated 32 times, as against 15 divisions for the controls. 

 Phelps's (1926) experiences with Chaos dijfluens were not in agree- 

 ment with those of Hartmann. Successive excisions of one-fourth of the 



