148 On the Genetics of the Clliate Protozoa 



environmental conditions, unlimited power of reproduction without con- 

 jugation or artificial stiiindation " (Woodruff, 1911 d). 



44. Concerning the morphological manifestations of " depression " 

 there is not complete unanimity among investigators. For whereas 

 some believe that depressed organisms are of smaller size (Maupas, 

 Calkins, Popoff), others maintain that they are larger (Woodruff, 

 Gregory) : and similarly some observers find an increase in the relative 

 size of the raeganucleus (Hertwig, Popoff, Sun), others a decrease 

 (Calkins, Moody, Woodruff). Some of these conclusions cannot be 

 accepted unreservedly. It seems on the whole probable that hyper- 

 trophy of the meganucleus often accompanies depression : and I have 

 little doubt that the discordant conclusions result in pjirt from the 

 loose application of the terms "depression" or "senescence" to a variety 

 of different conditions. 



4-5. We may now consider some remarkable work by Popoff (1908) 

 on what may be called the mechanics of division. The aim of his 

 investigations was to test the truth of one of R. Hertwig's deductions 

 from his " Kernplasmatheorie " (§ 30). According to Hertwig, the 

 normal karyoplasmic ratio of an organism (or cell) becomes gradually 

 altered owing to the growth of nucleus and cytoplasm at unequal rates, 

 until a condition of " karyoplasmic tension " is finally set up. This 

 tension, or unbalanced condition, is the factor which determines division 

 of the organism (or cell) as a whole — the division giving rise to new 

 individuals in which the karyoplasmic ratio is once more normal. 

 Division is thus regarded as a regulatory reaction to an abnormal 

 mass-relation between nucleus and cytoplasm. 



46. Popoff's earliest results (1908)' were obtained with Frontonia, 

 a large holotrichous ciliate of symmetrical and regular shape suitable 

 for measuring. It possesses a single meganucleus — upon which the 

 measurements were made — and several micronuclei. Cultures were 

 kept at various temperatures, and the volume of nucleus and cytoplasm 

 computed from measurements made on animals at known intervals of 

 time between one division and the ne.xt. The following results may be 

 selected as illustrating Popoff's general conclusions. (See Fig. 3.) 



47. At a temperature of 25' C. Fmntoiiia divides once every 

 17 hours, during which time the cytoplasm grows at a uniform rate 

 (Fig. 3). The nucleus, however, begins immediately after division to 

 decrease slightly in volume — probably owing to loss of liquid during 



' These experiments were beguu origiually by Wierzbizki, another of Hertwig's pupils 

 (Hertwig, 1905). 



