PRESIDENTIAL ADDRESP. 453 



both male and female parents and which maintain unimpaired their distinct 

 and specific individuality through the entire life-cycle. This distinctness is 

 apparent at least in the germ-cell-cycle of the organism, but may be obscured 

 by secondary changes in the nuclei of the specialised tissue-cells. 



Only in the very last stage of the life-cycle do the group of male and female 

 chromosomes modify their behaviour in a most striking manner. In the final 

 generation of oogonia or spermatogonia, from which arise the oocytes and 

 spermatocytes which in their turn produce the gamete-cells, it is observed that 

 the male and female chromosomes make a last appearance in their full number, 

 and then fuse in pairs, so as to reduce the number of chromosomes to half that 

 previously present. 



In Aggregata also Dobell and Jameson have shown that the union of the 

 pronuclei in fertilisation brings together two sets each of six chromosomes, 

 and that these then fuse with one another in pairs according to type, that is 

 to say a with a, h with h, c with c, and so on. Analogous phenomena have 

 lieen demonstrated also in the gregarine Diplospora. We have here a difference 

 in detail, as compared with the Metazoa, in that the fusion takes place at the 

 fertilisation and not as the first step in the maturation of the germ-cells ; but 

 in both cases alike the fusion of chromatin-elements individually distinct and 

 exhibiting specific characteristics is to be regarded as the final consummation 

 of the sexual act, though long deferred in the Metazoan life-cycle. 



As Vejdovsky has pointed out, there can be no more striking evidence of 

 the specific individuality of the chromosomes than their fusion or copulation 

 in relation to the sexual act. Is there any other constant element or constituent 

 of living organisms exhibiting to anything like the same degree the essentially 

 vital characteristics of individuality manifested in specific behaviour? If there 

 is, it remains to be discovered. 



I come now to the question of the permanence and immortality, in the 

 biological sense of the word, of the chromatinic particles, which may be sum- 

 marily stated as follows : the chromatinic particles are the only constituents 

 of the cell which maintain persistently and uninterruptedly their existence 

 throughout the whole life-cycle of living organisms universally. 



I hope I shall not be misunderstood when I enunciate this apparently sweep- 

 ing and breathless generalisation. I am perfectly aware that in the life-cycle of 

 any given species of organism there may be many cell-constituents besides the 

 chromatin-particles that are propagated continuously through the whole life- 

 cycle ; but cell-elements which appear as constant parts of the organisation of 

 the cell throughout the life-cycle in one type of organism may be wanting 

 altogether in other types. With the exception of the chromatin-particles there 

 is no cell-constituent that can be claimed to persist throughout the life-cycles 

 of organisms universally. To take some concrete examples ; the cytoplasmic 

 grains known as mitochondria or chrondriosomes have been asserted to be 

 persistent elements throughout the germ-cycle of Metazoa, and the function 

 of being the bearers of hereditary tendencies has been ascribed to them. But 

 Vejdovskj' ^' flatly denies the alleged continuity in cases investigated by him, 

 and though chrondriosomes have been described in some Protozoa, there is no 

 evidence whatever that they are of universal occurrence in Protista. Centro- 

 somes, intranuclear or extranuclear, have been stated to be constant cell- 

 components in some organisms ; whether that is true or not it seems quite 

 certain that in many organisms the cells are entirely without centrosomic bodies 

 of any kind, as for example in the whole group of Phanerogams. So it is 

 with any other cell-constituent that can be named. It may be that this is only 

 the result of our incomplete knowledge at the present time. I am prepared, 

 however, to challenge anyone to name or to discover any cell-constituent, other 

 than the chromatinic particles, which are present throughout the life-cycle, not 

 merely of some particular organism, but of organisms universally. 



In this feature of continuity the chromatin-constituents of the cell present 

 a remarkable analogy with the germ-plasm of Metazoa. Just as the germ- 

 cells of Metazoa go on in an uninterrupted, potentially everlasting series of 

 cell-generations, throwing off, as it were, at each sexual crisis a soma which is 

 doomed to but a limited lease of life, during which it furnishes a nutritive 



" L.e. pp. 77-89. 



