458 TRANSACTIONS OF SECTION D. 



system may be amplified and differentiated in various ways and the cytoplasm 

 acquires manifold powers of internal or external secretion. And finally the 

 cytoplasm contains enclosures of the most varied kind, some of them metaplastic 

 products of the anabolic or catabolic activity essential to the maintenance of 

 life, others of the nature of special cell-organs performing definite functions, 

 such as centrosomes, plastids, chromatophores, &c., of various kinds. 



With all the diverse modifications of the cytoplasmic cell-body the nucleus 

 remains comparatively uniform. It may indeed vary infinitely in details of 

 structure, but in principle it remains a concentration or aggregation of numerous 

 grains of chromatin supported on some sort of framework over which the grains 

 are scattered or clumped in various ways, supplemented usually by plastin or 

 nucleolar substance either as a cementing ground-substance or as discrete grains, 

 and the whole marked off sharply from^ the surrounding cytoplasm, with or 

 without a definite limiting membrane. There is, however, one point in whicli 

 the nucleus exhibits a progressive evolution of the most important kind. 1 

 refer to the gradual elaboration and perfection of the reproductive mechanism, 

 the process whereby, when the cell reproduces itself by fission, the chromatin- 

 elements are distributed between the two daughter-cells. 



The chromatin-constituents of the cell are regarded, on the view maintained 

 here, as a number of mimite granules, each representing a primitive inde- 

 pendent living individual or biococcus. To each such granule must be attributed 

 the fundamental properties of living organisms in general ; in the first place 

 metabolism, expressed in continual molecular change, in assimilation and in 

 growth, with consequent reproduction ; in the second place specific individu- 

 ality. As the result of the first of these properties the chromatin-granules, 

 often perhaps ultra-microscopic, may be larger or smaller at different times, 

 and they multiply by dividing each into two daughter-granules. As a result 

 of the second property, chromatin-granules in one and the same cell may exhibit 

 qualitative differences and may diverge widely from one another in their 

 reactions and effects on the vital activities of the cell. The chromatin- 

 granules may be either in the form of scattered chromidia or lodged in a 

 definite nucleus. When in the former condition, I have proposed the term 

 chromidiosome -■* for the ultimate chromatinic individual unit ; on the other 

 hand, the term chromiole is commonly in use for the minute chromatin-grains 

 of the nucleus. The terms chromidiosome and chromiole distinguish merely 

 between the situation in the cell, extranuclear or intranuclear, of the individual 

 chromatin-grain or biococcus. 



In the phase of evolution that I have termed the pseudomoneral or cytodal 

 phase, in which the organism was a droplet of periplasm containing scattered 

 biococci or chromidiosomes, metabolism would result in an increase in the size 

 of the cytode-body as a whole, accompanied by multiplication of the chromidio- 

 somes. Individualisation of the cytodes would tend to the acquisition of a 

 specific size, that is to say, to a limitation of the growth, with the result 

 that when certain maximum dimensions were attained the whole cytode iwould 

 divide into two or more smaller masses amongst which the chromidiosomes 

 would be partitioned. 



In the next stage of evolution, the protocyte with a definite nucleus, it is 

 highly probable that at each division of the cell-body, whether into two or 

 more parts, the primitive method of division of the nucleus was that which I 

 have termed elsewhere ' chromidial fragmentation ' ; °' that is to say, the 

 nucleus broke up and became resolved into a clump of chromidiosomes, which 

 separated into daughter-clumps from which the daughter-nuclei were recon- 

 stituted. Instances of nuclear divisions by chromidial fragmentation are of 

 common occurrence among the Protozoa and represent probably the most 

 primitive and direct mode of nuclear division. 



It is clear, however, that if the chromatin-grains are to he credited with 

 specific individuality and qualitative differences amongst themselves, this 

 method of nuclear division presents grave imperfections and disadvantages, 

 since even the quantitative partition of the. chromatin is inexact, while the 

 qualitative partition is entirely fortuitous. Chromidiosomes having certain 



-'' Introduction of the Study of tJic Protozoa, Arnold, 1912, p. 65. 

 " Oj). cit. p. 101. 



