CELL AND ORGANISM 



101 



tozoa than with the Protophyta, I shall 

 utilize information from the former rather 

 than the latter. The border line between 

 the two lies in the Flagellata in which the 

 two kingdoms, plant and animal, are 

 merged. It has been the custom for bota- 

 nists to regard groups containing species 

 with ehromatophores as in the plant king- 

 dom and for zoologists to return the com- 

 pliment by regarding all flagellates as ani- 

 mals. Morphologically the group is an 

 indivisible one. Some genera have species 

 on both sides of the fence and certain spe- 

 cies of Euglena belong to both and can be 

 shifted experimentally from one to the 

 other overnight. In the light their chromo- 

 plasts are green and their metabolism is 

 synthetic. In the dark in nutrient solu- 

 tions the chromoplasts turn to leucoplasts 

 and the metabolism becomes saprozoic. The 

 Dinoflagellata illustrate the intermediate 

 position of flagellates very amply. Many 

 of them have chromoplasts and a rigid cel- 

 lulose exoskeleton while a very large group, 

 less well known because of their fragile 

 bodies are completely holozoic, even can- 

 nibalistic, in their feeding habits, though 

 many of these animal representatives are 

 brilliantly colored. Still others have be- 

 come parasites in other Protozoa and in a 

 wide range of marine invertebrates. 



Both holozoic and holophytic flagellates, 

 as well as all other classes of Protozoa, tend 

 to become multicellular, in the simplest 

 cases, only briefly at the onset of fission. 

 In not a few others the retention in the or- 

 ganism of the products of division is per- 

 manent in a body of characteristic form 

 and behavior. Symmetry and behavior 

 alike assert their organismic individuality. 

 The popular superstition that the Protozoa 

 are unicellular and the Metazoa are multi- 

 cellular is fostered by the over-simplifica- 

 tion required for uninstructed beginners 

 in biology. As a sheer matter of fact the 

 organisms in each group are both unicellu- 

 lar and multicellular, but the time spent in 

 each state may be quite different. All 

 Metazoa are unicellular in the gamete and 

 fertilized egg stages, and all Protozoa have 

 at least a brief existence in the multicellu- 

 lar state and some of them repeatedly re- 



vert to it in the course of their complicated 

 life cycles. For example, the malarial par- 

 asite, Plasmodium vivax, living in the 

 stomach of the mosquito, in the outer walls 

 of the stomach, in the salivary fluid, and in 

 the red cells of blood of man, assumes the 

 multicellular condition in the sporoblast, 

 possibly retaining it during one or more 

 asexual fissions of that multicellular body, 

 and again in the multicellular trophozoite 

 in the red cell. It reverts to the unicellular 

 condition in the gametes, zygote, initial 

 trophozoite, in the oft-repeated merozoite 

 phases, and in the gametocytes. Through- 

 out this complicated life cycle the genetic 

 individual appears in a series of distinctive 

 unicellular individuals each of which 

 speedily becomes multicellular by growth 

 and cell multiplication within the body. 

 In a comparable manner, Paramecium 

 starts its life cycle as a single-celled zygote 

 nucleus in its great grandmother's cyto- 

 plasm, goes through division stages to an 

 eight-celled morula without cell boundaries 

 or cleavage of cytoplasm, differentiates 

 into sex and somatic cells, which are then 

 distributed by two binary fissions into the 

 two-celled adult. Yet all youngsters and 

 not a few oldsters continue to regard Para- 

 mecium as a unicellular protozoan. The 

 end result in the life cycle of certain Sporo- 

 zoa is a pansporoblast of six cells, 2 wall 

 cells, 2 thread cells, and 2 germ cells. For 

 purposes of dissemination to new hosts this 

 protozoan utilizes a multicellular body with 

 histological differentiation of tissues, of the 

 minimum possible number of cells to be 

 sure, but still along precisely comparable 

 lines to the processes of development in the 

 so-called multicellular Metazoa. The dif- 

 ferences between the Protozoa and the 

 Metazoa lie in numbers of cells attained 

 rather than in the principle of unicellular- 

 ity of the one and the multicellularity of 

 the other. 



The purely temporary nature of the mul- 

 ticellular stage in the primitive flagellates 

 is well illustrated in the genus Trichomonas, 

 a parasite of the digestive tract of many 

 vertebrates and some insects. It forms by 

 repeated mitoses multicellular plasmodia of 

 sixteen cells, but each has the full equip- 



