CELL AND ORGANISM 



99 



grouped cells without dividing walls, and 

 binucleate and multinucleate cells, so des- 

 ignated because there is no visible partition 

 of the respective zones of influence of each 

 of the nuclei. Thus Paramecium is habitu- 

 ally referred to in textbooks as a one-celled 

 animal and at the same time the two nuclei 

 are differentiated as sexual and somatic. 

 The culmination of this concept that sim- 

 plicity is a function of unicellular Proto- 

 zoa, and complexity of multicellular Meta- 

 zoa is the erratic idea that the Protozoa are 

 noncellular. Thus an extraordinarily com- 

 plicated Radiolarian with hundreds of ele- 

 ments in its skeleton and sixteen hundred 

 chromosomes in its single large nucleus 

 cannot be cellular; instead it is an organ- 

 ism. With comparable logic one should 

 postulate that the matured germ cell, and 

 the zygote after the fusion of the gamete 

 nuclei and before the first cleavage, are also 

 noncellular ! 



This seeming incongruity of regarding a 

 single cell as also an entire organism was 

 singled out by the late Professor Whitman 

 who in his Woods Hole lecture (1894) on 

 The Inadequacy of the Cell Theory invited 

 attention to the fact that the Cell Theory 

 postulated that all organisms are composed 

 of cells, and that in this form it made no 

 provision for organisms in the initial one- 

 celled stage and none for unicellular Pro- 

 tista. In other words, what by definition is 

 only a part could never logically be the 

 equivalent of the whole. This led to a 

 modification of the definition of the cell to 

 the simple statement that the cell is the 

 structural and functional unit of organic 

 structure; and that conversely all organ- 

 isms are composed of one or more cells, and 

 furthermore, that normally at some period, 

 namely, the starting point of their life 

 cycles, all organisms exist as one cell. In 

 this stage, alike in the phylogeny of the 

 plant and animal kingdoms and in the 

 ontogeny or life cycle of the genetic indi- 

 vidual, the union of cell and organism in 

 one living entity occurs. The cell is at that 

 time merged in the organism and is the 

 whole of it; and conversely, the whole or- 

 ganism consists of but a single cell. 



The physiological significance of the 



small sizes prevalent among the cells of 

 plants and animals has a bearing on the 

 problem of cell and organism. The living 

 substance is characteristically acquisitive. 

 Growth is a sign of life and the metabolic 

 rate records the speed of living. Acqui- 

 sition is greatest in the earlier part of the 

 life cycle and slows down with age or ad- 

 verse conditions, such as the lowering of 

 the temperature below the optimum. The 

 substances stored in the cell as a result of 

 acquisition are sources of energy which in 

 the living organism can be transformed 

 into the activities of life only in the living 

 cell. 



Since the initial stage in the life cycle of 

 all organisms is normally a single cell, we 

 may well ask why in the course of evolution 

 did not and does not the primal cell merely 

 increase in size and complexity and thus 

 create the larger organisms in the scale of 

 being and, in ontogeny, also increase the 

 zygote to adult size and complexity? 



A survey of cell size in the Protozoa is 

 instructive in this particular. There ap- 

 pear to be rather definite limits in size and 

 volume beyond which these animals do not 

 go, and somewhat similar limits seem to 

 prevail among the Protophyta. Protozoa 

 as small as 2 to 3 |j are known among flagel- 

 lates and Sporozoa. Reduction in size fol- 

 lows with rapid succession of asexual repro- 

 duction by fission. Most Protozoa are less 

 than 100 |j in the largest dimension. A few 

 Dinoflagellata of attenuate form may reach 

 a length of 1500 p. Multicellular stages of 

 Protozoa, as a rule, are larger than unicel- 

 lular. Some multicellular Radiolaria form 

 colonies of up to five or more millimeters 

 in diameter. 



The tissues of the Metazoa and Meta- 

 phyta exhibit limits in the sizes of their 

 constituent cells of approximately the same 

 order of magnitude as those in the more 

 primitive Protista, except for yolk-laden 

 ova, but even in these the amount of cyto- 

 plasm remains relatively small. 



The relations of nucleus and cytoplasm 

 within the cells also exhibit certain limita- 

 tions which Richard Hertwig sought to ex- 

 press in his nucleocytoplasmic ratio. In 

 many cells the ratio of the volume of the 



