PRESIUKNTIAL ADDRESS. ^ 441 



that a description in general terms is no longer possible. I shall deal with some 

 of these types later in my attempts to reconstruct the evolution and phylogeny 

 of the cell. I will draw attention now only to a few salient points. In the 

 Protist cell the chromatin is not necessarily confined to the nucleus, but may 

 occur also as extranuclear grains and fragments termed chromidia, scattered 

 through the protoplasmic body; and the chromatin may be found only in the 

 chromidial condition, a definite nucleus being temporarily or permanently 

 absent. Further, when a true nucleus is present in the Protist body, it seldom 

 contains a nucleolus of the same type as that seen in the nuclei of tissue-cells, 

 that is to say, a mass of pure plastin, but in its place is found usually a 

 conspicuous body which shows reactions agreeing more or less closely with those 

 of chromatin and which consists of a plastin-basis more or less densely impreg- 

 nated with chromatin. Such a body is termed a karyosome (or chromatin- 

 nucleolus) to distinguish it from the true nucleoli (plastin-nucleoli) characteris- 

 tic of tissue-cells. According as the plastin or the chromatin predominates in 

 the composition of a karyosome, its reactions may resemble more nearly those 

 of a true nucleolus in the one case, or those of chromatin in the other. The so- 

 called karyosomatic type of nucleus is very common in the Protista, but by no 

 means of invariable occurrence; in many cases the nucleus consists of a clxmip 

 of small grains of chromatin, with no distinct karyosome, or with a karyosome 

 which consists mainly of plastin. Thus two extreme types of nuclear structure 

 can be distinguished and may be termed provisionally the karyosomatic type 

 and the granular type, ignoring for the sake of convenience in nomenclature 

 the types of structure transitional between the two; as, for example, types in 

 which a distinct karyosome is seen together with more or fewer peripherally 

 arranged grains of chromatin. 



In either the karyosomatic or the granular type of Protist nucleus we may 

 find great simplification of the complex type of nuclear structure seen in the 

 tissue-cells of animals and plants. Thus in the first place a distinct nuclear 

 membrane may be entirely absent and the chromatin>-elements, whether occur- 

 ring in the form of a compact karyosome or of a clump of grains, are lodged 

 simply in a vacuole in the cytoplasm, that is to say in a cavity containing a 

 watery fluid of nuclear sap ini which the mass or masses of chromatin are sus- 

 pended. It is a moot point, to which I shall return again, whether in nuclei 

 of this simple type the linin-framework may sometimes be absent altogether, 

 or whether it is invariably present in at least a rudimentary form, appearing 

 as delicate threads (in optical section) extending from the chromatin-masses 

 to the limiting wall of the nuclear vacuole, or between the grains of chromatin 

 themselves. When such a framework can be detected, the nucleus acquires the 

 appearance, in preserved preparations at least, of possessing a definite structure 

 and is often termed a resting nucleus ; many observations have shown, how- 

 ever, that the nucleus during life is undergoing continual internal movements 

 and re-arrangements of its parts and is by no means at rest. The linin-frame- 

 work cannot, therefore, be regarded in any way as a rigid skeleton, but must 

 be interpreted as an alveolar framework similar to that of the general protoplasm 

 and equally liable to movement, displacement, and change. 



From this survey, necessarily most brief and superficial, of the manner in 

 which the nuclei of Protists may vary from the type of nucleus described in 

 the text-books, it is at once evident that the essential part of the nucleus is 

 the chromatin, and that the other structural constituents of the nucleus, namely, 

 membrane, framework, and plastin or nucleolar bodies, are to be regarded as 

 accessory components built up round, or added to, the primary nuclear material, 

 the chromatin Even with regard to the nuclei of Metazoa it is maintained 

 by Vejdovsky that at each cell-generation the entire nucleus of the daughter- 

 cell is produced from the chromosomes alone of the mother cell.' The simplest 

 body which can be recognised as a nucleus, distinct from the chromidia scat- 



' Walker, on the other hand, considers that 'it seems quite possible that 

 the chromatin is merely a seci-etion of the linin.' (Science Proyrcsa, vol. vii. 

 P- 641.) I doubt ■whether there are many cytologist.'? who would admit this 

 possibility, and I think that very few protistologista would assent to any 

 such notion, since in the nuclei of Protista the linin-framcwovk is in many cases 

 very little in evidence, if present at alt. 



