266 BIOLOGY OF THE PROTOZOA 



the cell. It is quite possible that this divisional reorganization 

 is adequate for the preservation of the protoplasm through long 

 periods of activity and may be the explanation of the long-continued 

 life in certain cultures of ciliates, or continued life of animal flagel- 

 lates in which fertilization processes are unknown. 



Other differentiations occur in Protozoa which cannot be regarded 

 as inter-divisional in character. These are rather of a cumulative 

 nature and are not lost with the de-differentiation which occurs at 

 division. 



2. Cyclical Differentations.— This second group of differentia- 

 tions is not manifested in every cell of a species but appears at 

 certain phases in the life history of the protoplasm composing any 

 series of individuals. They are racial, therefore, and correspond 

 roughly w T ith periods in metazoon development such as youth, ado- 

 lescence and age. Some of these differentiations are characteristic 

 of very young forms, occurring immediately after fertilization and 

 at no other time in the life cycle. Others make their appearance 

 later in the cycle and often after many generations by division. 

 These lead to and accompany the phenomena of fertilization and 

 include maturation stages and gamete formation. Still others occur 

 only at the end phases of the life cycle and are specific characteristics 

 of age. We find justification, therefore, for purposes of description 

 at least, in presenting facts concerning differentiations of youth, 

 of maturity and of age, but we have no intention of setting limits 

 to these phases. 



(a.) Cyclical Differentiations Peculiar to Youth. — Intensity of 

 metabolic activities is one of the characteristic features of young 

 organisms, but with Protozoa exact data are difficult to get except 

 from isolation cultures. In such cultures intensity is indicated by 

 the division-rate and the great majority of ciliates show a higher 

 division-rate in the early periods of vitality (see p. 250 and Figs. 

 131 to 133). In Urolcptus mobilis this intensity lasts for approxi- 

 mately sixty days (Fig. 131) and in Spathidium spathula for about 

 forty days (Fig. 133). The evidence is not consistent, however, if 

 all isolation cultures are considered, and in exceptional cases of 

 Uroleptus and of Spathidium there is no indication of this relative 

 intensity. Nor does Belaf give any evidence of it in his isolation 

 cultures of Actinophrys sol; nor does Hartmann (1921) for Eudorina 

 eUgans, nor E. and M. Chatton (1923-1925) for Glaucoma scintil- 

 lans. In such cases it is quite possible that the conditions of the 

 cultures are such that differentiations are offset and reorganization 

 at division periods is adequate for continued vitality. Y\ ith para- 

 sitic forms exact data in this matter are wanting and general 

 impressions are of little value. 



Young organisms show the effects of abnormal conditions of the 

 environment more quickly and more intensely than do older ones. 



