Section XI 

 THE DETERMINATION OF SIZE 



N. J. BERRILL 



The size that any organism finally attains 

 is the result of growth, and the regulation 

 of size is essentially a matter of rate and 

 duration of growth. In most cases the greater 

 part, but not the greatest rate, of growth 

 occurs after the end of the embryonic period 

 proper, during the later phases of the whole 

 developmental cycle when there is compara- 

 tively little change of form. 



Growth, and therefore size regulation, is 

 associated with all things living, from the 

 sub-microscopical molecular components of 

 a cell to the giant organismal whales and 

 redwoods. Accordingly, any restriction of 

 growth- and size-analysis to certain organ- 

 isms or organizational levels must be purely 

 arbitrary. The basic phenomenon of growing 

 to a limited size is a general characteristic 

 of organisms and their parts, and above the 

 molecular level at least there seems to be 

 no such thing as a unit of growth. The cell 

 appears to be primarily a metabolic unit 

 and not in any fundamental sense either a 

 growth or an organizational unit. (Berrill, 

 '41; Sinnott, '45). 



Specific size, implying regulation of 

 growth, is as characteristic of individual 

 cells and single-celled organisms as it is of 

 multicellular organisms and of populations 

 of all kinds. Protozoa vary in size from the 

 numerous kinds only 2 or 3 micra long, to 

 the relative giants such as certain species of 

 Spirostomvim, Stentor, Paramecium and 

 Ameba several millimeters long, that is, a 

 range in linear dimensions of several thou- 

 sand times. The general problem of cell 

 size is probably best approached with such 

 organisms as these that are comparable to 

 cells but which have an independent exist- 

 ence, so being more amenable to culture 

 and experiment. Within the limits of a single 

 genus, Paramecium appears to offer the 

 greatest range in size, and it has already 

 been subject to extensive genetical investi- 

 gations. This approach, however, has far 



outrun the physiological; the modification of 

 size through metabolism control, including 

 temperature effects, is susceptible to a much 

 more intense experimental analysis. 



In multicellular organisms the sizes of 

 cells, other than eggs, vary only between 

 certain relatively narrow limits, and while 

 they are by no means everywhere the same, 

 body size is primarily a reflection of cell 

 number rather than cell size. Tissue for 

 tissue, the cells of mouse and elephant vary 

 about 1:2 in linear dimension, cells are of 

 the same size in the 9-foot leaf of the giant 

 water lilies and in the small, while in dwarf 

 and giant species of the slipper-limpet Crepid- 

 ula, Conklin ('12) found equivalent cell 

 types to be of virtually the same size. 



The exceptions to this general condition 

 are significant. Compared with other ver- 

 tebrates the amphibians have relatively large 

 cells, yet within this group the perenni- 

 branchiate urodeles have practically all cell 

 types many times the size of their metamor- 

 phosing cousins, erythrocytes for example 

 reaching as much as 70 micra, larger than 

 the eggs of some animals. These forms are 

 also distinguished by a comparatively low 

 rate of metabolism (Smith, '12), a correlation 

 that merits further investigation. 



Cells generally have the diploid number 

 of chromosomes, but under certain circum- 

 stances may be haploid or polyploid. The 

 cell size varies directly with these nuclear 

 conditions, though not proportionately. With- 

 in the limits investigated, organismal and 

 organ sizes in heteroploid amphibian larvae 

 remain unaffected, the cell number in some 

 way becoming adjusted (Fankhauser, '45). 

 In certain insects such as Culex the cells, 

 e.g., intestinal epithelium, become progres- 

 sively polyploid and enlarge as the larva 

 grows. With the onset of metamorphosis they 

 undergo a series of divisions leading to an 

 imago with a reconstituted gut consisting of 

 relatively small and numerous diploid cells 



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