GROWTH 525 



time. To these values were added successively the average increments, in 

 order to obtain the values at each time. The sum of the nuclear and the 

 cytoplasmic volumes is plotted in Figure 132. The increments were 

 read from Popoff's summary graph. 



The growth in volume of F. leucas is more rapid during the first hour; 

 the rate then decreases, until the relative rate is nearly linear, that is, 

 until about one hour before the next division. The rate increases before 

 division, mainly owing to the increase in nuclear volume. The volume 

 of the cytoplasm increases continually. The volume of the nucleus de- 

 creases for the first two hours after division to about 86 percent of its 

 size at fission, then increases slowly, until it shows the customary rapid 

 increase in the three hours preceding a new fission. 



Entz (1931) has followed the growth of populations and of indi- 

 vidual dinoflagellates, Ceratium hirudinella, in their natural habitats. 

 The largest animals were found in April. The highest rate of division 

 occurred in late June, July, and August, which coincides with the period 

 of maximum temperature. The growth was measured in three dimen- 

 sions, and the time, in hours, for the growth stages is : nucleal and cyto- 

 plasmic division, one; horn regeneration, 2; slim stage, 27; indifferent 

 stage, 72; compacted stage, 18; total, 120 hours. 



Extensive studies have been made of the variation in the sizes of 

 Protozoa, and this information has been collated by Adolph (1931). 

 A change in form during growth is reported by MacLennan (1935) for 

 Ichthyophihirius. This and other Protozoa would furnish excellent ma- 

 terial for the study of allometric growth. Cf. Huxley (1932), Need- 

 ham (1934), Teissier (1934), Richards (1935), and Huxley and Teis- 

 sier ( 1936) . The size of Paramecium hursar'ia was modified by changing 

 the pH of the culture medium (Loefer, 1938a). Cell size and nuclear 

 size in Oxytricha jallax was found by Woodruff (1913a) to be least 

 during the periods of rapid reproduction, and to become larger as the 

 division rate decreases. The nucleocytoplasmic ratio was highest during 

 the period of greatest reproductive activity, and this was interpreted as 

 an incidental result rather than as a cause of the rate of division. 



The first evidence of division is a slight groove encircling the animal, 

 and separation occurs in Paramecium about one-half hour later (Jen- 

 nings, 1908). The nucleocytoplasmic ratio increases for the first three 

 hours, and then decreases for the next three hours (Popoff, 1909). 



