II] OF THE LEAST OF ORGANISMS 63 



units, in place of the many thousands which make up such an organ 

 in larger, if not alwayc higher, animals. We have already spoken 

 of the Fairy-flies, a few score of which would hardly weigh down 

 one of the larger rotifers, and a hundred thousand would weigh less 

 than one honey-bee. Their form is complex and their httle bodies 

 exquisitely beautiful; but I feel sure that their cells are few, and 

 their organs of great histological simplicity. These considerations 

 help, I think, to shew that, however important and advantageous 

 the subdivision of the tissues into cells may be from the construc- 

 tional, or from the dynamic, point of view, the phenomenon has 

 less fundamental importance than was once, and is often still, 

 assigned to it. 



Just as Sachs shewed there was a hmit to the amount of cytoplasm 

 which could gather round a nucleus, so Boveri has demonstrated 

 that the nucleus itself has its own hmitations of size, and that, in 

 cell-division after fertilisation, each new nucleus has the same size 

 as its parent nucleus*; we may nowadays transfer the statement 

 to the chromosomes. It may be that a bacterium lacks a nucleus 

 for the simple reason that it is too small to hold one, and that the 

 same is true of such small plants as the Cyanophyceae, or blue-green 

 algae. Even a chromatophore with its "pyrenoids" seems to be 

 impossible below a certain sizej. 



Always then, there are reasons, partly physiological but in large 

 part purely physical, which define or regulate the magnitude of the 

 organism or the cell. And as we have already found definite 

 Hmitations to the increase in magnitude of an organism, let us now 

 enquire whether there be not also a lower limit below which the 

 very existence of an organism becomes impossible. 



* Boveri, Zellen.studien, V: Ueber die Abhangigkeit der Kerngrosse und 

 Zellenzahl von der Chromosomenzahl der Ausgangszellen. Jena, 1905. Cf. also 

 {int. al.) H. Voss, Kerngrossenverhaltnisse in der Leber etc., Ztschr.f. Zellforschung, 

 VII, pp. 187-200, 1928. 



t The size of the nucleus may be affected, even determined, by the number of 

 chromosomes it contains. There are giant race* of Oenothera, Primula and Solanum 

 whose cell-nuclei contain twice the normal number of chromosomes, and a dwarf 

 race of a little freshwater crustacean, Cyclops, has half the usual number. The 

 cytoplasm in turn varies with the amount of nuclear matter, the whole cell is 

 unusually large or unusually small; and in these exceptional cases we see a direct 

 relation between the size of the organism and the size of the cell. Cf. {int. al.) 

 R. P. Gregory, Proc. Camb. Phil Soc. xv, pp. 239-246, 1909; F. Keeble, Journ. 

 of Genetics, ii, pp. 163-188, 1912. • 



