PRESIDENTIAL ADDRESS. 439 



organisation than that of the single ceU. The exploitation, if I may Tise the 

 term, of the Protista, though still in its initial stages, has already shown that 

 it is amongst these organisms that we have to seek for the forms which indicate 

 the evolution of the cell, Iwth those linos of descent which lead on to the cell 

 as seen in the Metazoa and jNletaphyta, as well as other lines leading in direc- 

 tions altogether divergent from the typical cell of the text-book. We find in 

 the Protista every possible (tondition of structural differentiation and elabora- 

 tion, from cells as highly organised as those of IMetazoa or even, in some 

 cases, much more so, back to iypes of structure to which the term cell can only 

 be applied by stretching its meaning to the breaking-point. Already one 

 generalisation of cytologists has been torpedoed by the study of the Protista. 

 The dictum ' Omnis nucleus e nucleo ' is perfectly valid as long as it is 

 restricted to the cells of Metazoa and Metaphyta, to the material, that is to 

 say, to which the professed cytologist usually confines his observations.* But 

 in the Protista it is now well established that nuclei can arise dc novo, not 

 from pre-existing nuclei but from the extranuclear chromatin for which 

 Hertvi-ig first coined the term 'chromidia.' 



It is clear, therefore, that the results already gained from the study of the 

 Protista have brought about a new situation which must be faced frankly and 

 boldly. It is impossible any longer to regard the cell as seen in the Metazoa 

 and as defined in the text-books as the starting-point of organic evolution. It 

 must be recognised that this type of cell has a long history of evolution behind 

 it, which must be traced out, so far as the data permit. The construction of 

 phylogenies and evolutionary series is of course purelj^ speculative, since these 

 theories relate to events which have taken place in a remote past, and which can 

 only be inferred dimly and vaguely from such fragments of wreckage as are to 

 be found stranded on the sands of the time in which we live. Many 

 important stages of evolution may be totally submerged and no longer available 

 for study and consideration. The extent to which such speculations will carry 

 conviction to a reasonable mind will depend entirely on the stores of data that 

 can be collected and which must be the last appeal for the cogency of all argu- 

 ments and judgments. The study of the Protista is as yet in its infancy; 

 groups have been recognised and have received ponderous designations although 

 their very existence is yet in doubt, as in the case of the so-called Chlamydozoa ; 

 and our knowledge of the affinities and mutual relationships of the groups is still 

 very imperfect. All attempts, therefore, to trace the evolution of the Protista 

 must be considered as purely tentative at present. If I venture upon any such 

 attempt, it is to be regarded as indicating a firm belief on my part that the 

 evolution of the cell has taken place amongst the Protista, and that its stages 

 can be traced there, rather than as a dogmatic statement that the evolution has 

 taken place in just the manner which seems to me most probable. When we 

 reflect on the irreconcilable differences of opinion amongst zoologists with regard 

 to the origin and ancestry of vertebrates, for example, we may well be cautious 

 in accepting pedigrees in Protista. 



Before, however, I can proceed to deal with my main subject, it is abso- 

 lutely necessary that I should define clearly the sense in which I propose to use 

 certain terms, more especially the words 'cell,' 'nucleus,' 'chromatin,' 'proto- 

 plasm,' and ' cytoplasm.' Unless I do so my position is certain to be mis- 

 understood, as, indeed, it has been already by some of my critics. 



The term cell was applied originally by botanists to the single chambers or 

 units of the honeycombed structure seen in the tissues of plants. The applica- 

 tion of the term to such structures is perfectly natural and intelligible, since 

 each such cell in its typical form is actually a closed space limited by firm 



* Vejdovsky {Zitm Prohlem der Vererhwnqsfraqer, Prag, 19111912, p. 120) 

 has already maintained, for the cells of Metazoa, that Fleming's aphorism 

 ' Omnis nucleus e nucleo ' should be changed to ' Omnis nucleus e chromoso- 

 matis ' [sic], on the ground that the nucleus, as such, is not an original cell- 

 component ' but is produced secondarily from the chromosomes of the mother- 

 cell.' If this is true, there is but little difference in detail, and none in 

 principle, between the formation of ' secondary ' nuclei from chromidia and 

 the reconstruction of a daughter-nucleus from chromosomes in the most perfected 

 form of karyokinesis. 



