578 THE LIFE CYCLE 



units into which they split are single cells, instead of flagellated cham- 

 bers or axial organizers. 



The Life Cycle of Plasmodium vivax 



A second example of the same general pattern, with added speciali- 

 zations due to parasitism in two hosts, is found in the malarial parasite, 

 Plasmodium vivax, with the sexual and one (or two?) asexual phases 

 in the mosquito. Anopheles, and oft-repeated merogony in the red cells 

 of the blood of man, and a second asexual phase in the gamete mother 

 cells of the male only on transfer of these cells to a lower temperature 

 than that of the blood of man, as on a microscope slide or in the stomach 

 of the mosquito. 



The Life Cycle of Paramecium caudatum 



The life cycle of Paramecium caudatiwi makes a definite evolutionary 

 advance in the Ciliata in two mutually interdependent features. The 

 first is the differentiation of sex and somatic cells in the same individual, 

 and the second is a permanent multicellular condition of two cells, de- 

 rived from an undifferentiated eight-celled cleavage stage. 



The original description of the cell, the selection of its name, the 

 focusing of attention on total cleavage, with plasmotomy in embryology, 

 rather than upon mitosis, all have combined to emphasize the separation 

 of one cell from another by a wall or dividing structural boundary. These 

 are all minor considerations. On the other hand, the significance of 

 derivation, continuity in time, physiological functions, and above all of 

 genetics, focus attention on the nucleus and the cytoplasm associated 

 with it or brought in in the normal sequence of growth, fertilization, 

 appropriation, or experiment under its control. These are all major con- 

 siderations. In this modern sense it is biologically medieval to refer, as 

 do many textbooks and other works, to Paramecium as a unicellular 

 organism. It is biologically quite as logical to call a whale unicellular. 

 Both start their cycles as one cell and both achieve multicellularity. No 

 great biological significance attaches to the particular number of cells 

 in the multicellular body, except during maturation. The significant 

 achievement is the differentiation of sex and somatic cells. One of the 

 primary distinctions in function, as well as in embryological origin, in 

 the multicellular metazoan is this differentiation. 



