170 PROTOZOOLOGY 



a long chain of generations indefinitely. Jennings (1929, 1942) who 

 has recently reviewed the whole problem states: "Some Protozoa 

 are so constituted that they are predestined to decline and death 

 after a number of generations. Some are so constituted that decline 

 occurs, but this is checked or reversed by substitution of reserve 

 parts for those that are exhausted; they can live indefinitely, but 

 are dependent on this substitution. In some the constitution is such 

 that life and multiplication can continue indefinitely without visible 

 substitution of a reserve nucleus for an exhausted one ; but whether 

 this is due to the continued substitution, on a minute scale, of re- 

 serve parts for those that are outworn cannot now be positively 

 stated. This perfected condition, in which living itself includes con- 

 tinuously the necessary processes of repair and elimination, is found 

 in some free cells, but not in all." 



Regeneration 



The capacity of regenerating the lost parts, though variable 

 among different species, is characteristic of all Protozoa from simple 

 forms to those with highly complex organizations, as shown by ob- 

 servations of numerous investigators. Brandt (1877) studied regen- 

 eration in Actinosphaerium eichhorni and found that only nucleate 

 portions containing at least one nucleus regenerated, and enucleate 

 portions or isolated nuclei degenerated. Similarly Gruber (1886) found 

 in Amoeba proteus the nucleate portion regenerated completely, 

 while enucleate part became rounded and perished in a few days. 

 The parts which do not contain nuclear material, may continue to 

 show certain metabolic activities such as locomotion, contraction of 

 contractile vacuoles, etc., for some time; for example, Grosse-Aller- 

 mann (1909) saw enucleate portions of Amoeba verrucosa alive for 

 20 to 25 days, while Stole (1910) found enucleate Amoeba proteus 

 living for 30 days. Clark (1942, 1943) showed that Amoeba proteus 

 lives for about seven days after it has been deprived of its nucleus. 

 Enucleated individuals show a 70 per cent depression of respiration 

 and are unable to digest food due to the failure of zymogens to be 

 activated in the dedifferentiating cytoplasm. It is now a well estab- 

 lished fact that when a protozoan is cut into two parts and the parts 

 are kept under proper environmental conditions, the enucleated 

 portion is able to carry on catabolic activities, but unable to under- 

 take anabolic activities, and consequently degenerates sooner or 

 later. 



In Arcella (Martini; Hegner) and Difflugia (Verworn; Penard), 

 when the tests are partially destroyed, the broken tests remain un- 



