444 TRANSACTIONS OF SECTION D. 



in the cytoplasm, stained in the same mamier as the chromatin of the nuclei. 

 Are these extranuclear granules to be regarded also as chromatin ? Yes, most 

 decidedly, because many laborious and detailed investigations have shown that 

 from this ring of granules in Arcella nuclei can arise, usually termed 

 ' secondary ' nuclei for no other reason than that they arise de novo from the 

 extranuclear chromatin and quite independently of the ' primary ' nuclei. 

 The secondary nuclei are, however, true nuclei in every respect, as shown 

 by their structure, behaviour, and relations to the life-history of the organism ; 

 they may fuse as nuclei of gametes (pronuclei) in the sexual act and they 

 become, with or without such fusion, the primary nuclei of future generations 

 of Arcella; they then divide by karyokinesis when the organism reproduces 

 itself in the ordinary way by fission, and are replaced in their turn by new 

 secondary nuclei at certain crises in the life-history. In view of these facts 

 it can be asserted without hesitation that the ring of staining granules in 

 Arcella is composed of, or at least contains, true chromatin-grains, extranuclear 

 chromatin for which R. Hertwig's term chromidia is now used universally. 

 It is interesting to note that until the life-history of Arcella was studied in 

 recent times the conspicuous ring of chromidia was generally overlooked and 

 is not shown in some of the older pictures of the organism. 



If, on the other hand, I make a preparation of some unidentified amceb.i 

 occurring casually in pond-water or in an infusion, and find in its cytoplasm 

 certain grains staining in same mamier as the chromatin of the nucleus, it is 

 quite impossible, without a knowledge of the life-history of the organism, to 

 assert definitely that the grains in question are or are not true chromidia. 

 They might equally well turn out to be volutin or any other substance that 

 has an affinity for the particular chromatin -stains used in making the 

 preparation. 



The fact that at the present time the only decisive criterion of what is 

 or is not chromatin is supplied only by its behaviour in the life-history and its 

 relation to the organism, makes it much easier to identify the chromatin in 

 some cases than in others. In those Protista or cells wliich contain, during 

 the whole or a part of the life-history, one or more true nuclei, recognisable 

 as such unmistakably by their structure and their characteristic relations to 

 the reproductive and sexual phenomena of the organism, the chromatin can 

 be identified with certainty. If chromidia occur in the cell-body in addition 

 to true nuclei, or even if the nuclei are temporarily ^absent during certain 

 crises of the life-history and the chromatin occurs then only in the form of 

 chromidia, there is still no difficulty in identifying the scattered chromatin- 

 grains by the fact that they contribute, soon or later, to the formation of 

 nuclei. 



On the other hand, in the simplest Protist organisms which do not contain 

 definite, compact nuclei recognisable by their structure and behaviour, the 

 identification of the chromatin may become correspondingly difficult. In the 

 absence of defitiite chemical criteria the term chromatin acquires then a 

 greater or less degree of vagueness and uncertainty of application, and it is 

 not easy to avoid a tendency to a pefitio princijni in attempting to define or 

 identify it. To a large extent we are thrown back upon the staining- 

 reactions, which I have already shown to be very unreliable, backed up by 

 analogies with those forms which possess definite nuclei. Since in the cells of 

 all animals and plants, and in all Protista which possess a true nucleus, the 

 chromatin is the one constituent which is invariably present, as I shall point 

 out in more detail subsequently, there is at least a strong presumption, though 

 not of course amounting to absolute proof, that it is present, or at least is 

 represented by some similar and genetically homologous constituents, in the 

 forms of simpler structure also. If then in Protista of primitive type we 

 find certain grains which exhibit the characteristic staining-reactions of 

 chromatin to be constantly present in the organism, grains which grow and 

 divide as a preliminary to the organism multiplying by fission and which are 

 partitioned amongst the daughter-organisms during the process of fission, so 

 that each daughter-individual reproduces the structure of the parent-form 

 from -which it arose; then there is very strong prima facie evidence, to say 

 the least, for regarding such grains ae homologous with the chromatin-grains of 

 ordinary cells. 



