VITALITY 483 



always involve changes in organization and for purposes of descrip- 

 tion it is convenient to describe them as : (1) Inter-divisional or 

 Ontogenetic Differentiations; (2) Cyclical Differentiations. 



1. Inter-divisional Differentiations.— In the development of a 

 ]\Ietazoon differentiated structures are never present in the initial 

 egg cell but appear in orderly sequence as a result of metabolism, 

 growth and division of cells. A protozoon about to emerge from 

 its cyst is comparable with such an egg cell. The cyst wall becomes 

 permeable, water and oxygen are admitted and metabolism begins. 

 Soon the characteristic motile organs make their appearance differ- 

 entiated from the apparently homogeneous protoplasm. The oral 

 apparatus, anal aperture, and contractile vacuole appear and the 

 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 V), 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 hy 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 Acanthacystis or of NodUuca 

 has none of the adult characters but develops them gradually 

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

 gregarine slowly acciuires, with growth, the particular epimerite, 

 protomerite, and deutomerite of its species (Fig. 122). Differ- 

 entiation occurs here but more slo-wly 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 

 leicisi for example, passes through stages resembling Leptomonas 

 and Crithidia (Fig. 118) or Leishmania donorani 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 develojjs tentacles and a 

 stalk before it becomes the adult form of the specific description. 

 Again, the young stages of a Blastodinimn are t^^^ically dinoflagellate 

 in character but the embryos develop into the peculiar parenchy- 

 matous sac-like adult quite unlike the usual dinoflagellate. 



Ontogenetic differentiations combined with growth and cell divi- 

 sion are evident in several of the colonial aggregates, particularly in 



