VITALITY 261 



organism emerges apparently complete from its cyst. This is a 

 rapid differentiation accompanying the onset of metabolism. 



Analogous processes of differentiation accompany the regenera- 

 tions associated with division of the cell. In ciliates a new oral 

 apparatus and specialized motile organs are formed at appropriate 

 positions by the dividing organism (see Chapter VI), and differ- 

 entiation is rapid and complete. The organization under which this 

 differentiation occurs is evidently a result of metabolic activities 

 prior to division (see below). 



Differentiations accompanying growth of the cell are characteristic 

 of Protozoa which reproduce by unequal or by multiple division. 

 Here the protoplasm is parcelled out amongst many offspring and 

 each bit of protoplasm, like an encysted cell or a cut-out fragment, 

 possesses the fundamental organization characteristic of the species, 

 but undifferentiated. Thus a bud of Acanthocystis or of a Suctorian 

 has none of the adult characters but develops them gradually 

 during a period of some days. Or the sporozoite of a polycystid 

 gregarine slowly acquires, with growth, the particular epimerite, 

 protomerite and deutomerite of its species (Fig. 126). Differentia- 

 tion occurs here, but more slowly than in the case of a ciliate, and 

 is apparently more directly associated with metabolism. Arrested 

 stages in development are not uncommon and frequently lead to 

 puzzling complications in the life cycle. Trypanosoma lewisi, for 

 example, passes through stages resembling Leptomonas and Crithidia 

 (Fig. 122) or Leishmania donovani through a flagellated Leptomonas 

 stage to an adult quiescent intracellular phase. Similarly the 

 young ciliated bud of a Suctorian which may be either parasitic or 

 free-living gradually loses its cilia develops tentacles and a stalk 

 before it becomes the adult form of the specific description. 



The changes in form and structure with growth are to be traced 

 to changes in the protoplasmic organization which in turn are 

 doubtless due to metabolic activities, and there is evidence that 

 analogous changes are responsible for the differentiations which 

 accompany regeneration in the more actively developing ciliates. 

 In this connection the merotomy experiments of Calkins (1911) 

 and Young (1922), patterned after the original merotomy experi- 

 ments of Balbiani (1891), are suggestive; in Chapter VI it is shown 

 that anticipatory changes in the cell precede the nuclear changes. 

 This was first demonstrated by Wallengren (1900) for Stylonychia 

 and Euplotes, and is clearly shown in Uronychia transfuga in which 

 the new posterior giant cirri are formed sometime prior to the 

 nuclear changes in preparation for division. The new cirri appear 

 in a region of the cell previously free from cirri, as well as at the 

 bases of the old cirri. Similarly there is a complete new formation 

 of the peristome with membranelles in the posterior half and a new 

 series of membranelles which replace the old ones in the anterior 



