526 



NA TURE 



[September 21, 1905 



Butschli has shown that the central portion of the con- 

 tents of the cell exhibit a foam structure in which granules 

 of a chromatin nature are embedded ; this is surrounded 

 by a thin laver of a less deeply stained substance, which 

 sometimes accumulates more prominently at the ends of 

 the cell. The central, more deeply stained, froth-like 

 structure with its granules is the nucleus ; the delicate 

 peripheral laver is the cytoplasm. From a recent examin- 

 ation which' I have made of BeggiaXoa alba, Beggwtoa 

 roseo-persicina, Bacillus siibtilis, and other smaller species, 

 I cannot agree with Butschli that there is a differentiation 

 into a central body or nucleus, and peripheral cytoplasm. 

 In the various species of Beggiatoa and Spirilla which 

 I have examined the cell contents exhibit a reticulate or 

 foam structure of the cytoplasm in which one or more 

 deeply stained granules may be embedded. As these 

 granules stain deeply in nuclear stains, and also give a 

 reaction for phosphorus, they are probably similar to 

 chromatin. They are distributed throughout the whole 

 cell, and are not specially confined to one place. 



We must conclude that the bacteria do not contain 

 anything which can be individualised as a nucleus, but 

 that the nuclein constituent of the cell when present is 

 contained in granules distributed throughout the cyto- 

 plasm. 



The Evolution of the Nucleus. 

 All plant nuclei, from the algae and fungi upwards, 

 present a striking similarity both in structure and rnode 

 of division. The same appears to be true of the animal 

 kingdom, from the protozoa upwards. But among the 

 protozoa on the animal side, and the yeast fungi, bacteria, 

 and Cyanophycea; on the plant side, there is a kind of 

 border kingdom in which occur structures which appear 

 to represent the nuclei of the higher organisms, but are 

 so different from them in many respects that it is very 

 difficult to say whether they should be regarded as nuclei 

 or not. As we have already seen, the central body of 

 the Cyanophycese and the chromatin granules of the yeast 

 plant and bacteria may represent simple or rudimentary 

 forms of nuclei. It is, therefore, possible that we may 

 obtain from them a clue or indication of some kind as 

 to the origin of the nucleus and the process of its evolution. 

 It is among the protozoa that we find the greatest 

 variation both in form and structure of these rudimentary 

 nuclei. All the various parts of the nuclei of the higher 

 animals can be recognised in them, but, as Calkins points 

 out, are rarely present in one and the same nucleus. From 

 a consideration of the various types Calkins considers that 

 the most primitive nucleus is probably a single mass of 

 chromatin without membrane or reticulum. By the 

 division of this into granules, their association into lines 

 forming primitive chromosomes, the development of a linin 

 network, and the formation of a definite nuclear membrane 

 was gradually brought about the development of the typical 

 nucleus. 



In the three groups of plants the Cyanophyceae, bacteria, 

 and yeast fungi it is not possible to recognise all the 

 various parts of typical nuclei as in the protozoa. In none 

 of them do we find a nuclear membrane, nucleolus, chromo- 

 somes, or spindle figure, or centrosome. We have nothing 

 very tangible, therefore, to compare with the typical 

 nucleus of the higher plants, and it is no doubt very largely 

 due to this that we have so many contradictory accounts 

 of the nuclear structures in these forms. 



At the same time the nuclei of the higher plants pass 

 through stages in their division which more nearly 

 approach in their structure the simple forms with which 

 we are now concerned. Thus the nuclear membrane and 

 nucleolus disappear, and the chromatin network becomes 

 condensed into a number of homogeneous rods or granules, 

 the chromosomes, which lie free in the cytoplasm. There 

 seems to be no reason why we should not consider the 

 simpler chromatin structures in the lower plants in the 

 light of these ontogenetic changes, as we may term them, 

 of typical nuclei, in order to obtain some indication of 

 the origin and phylogenetic development of the nucleus. 



We may take the colourless Beggiatoa as a starting- 

 point ; not that this form is the lowest, but because its 

 structure is, on account of its size, more easily examined, 

 and because it is connected possibly with the Cyanophyceae 

 on the one hand and with the ordinary bacteria on the 



other. From a careful examination and comparison of 

 its structure with that of other low forms we obtain the 

 following diagram, showing their possible relationships as 

 indicated by their cytological structure : — 



Beggiatoa alba. 

 (Chromatin granules may or may not be present.) 



Beggiatoa roseo-persicina, 

 Chromatium okenii, &c. 



Many chromatin granules 

 often condensed in the 

 centre of the cell. 

 Cytoplasm coloured. 



Cyanophyceae. 

 Central body with chro- 

 matin granules surrounded 

 by a peripheral coloured 

 protoplasm. 



I 

 ? 



■| 

 Algae. 



Larger Spirilla with 

 colourless cytoplasm. (Few 

 small chromatin granules). 



Forms like Cholera 

 vibrio, B. lineola, B. 

 anthracis. Typhus bacillus, 

 &c. Colourless. (One or 

 two chromatin granules.) 



Yeast Fungi. 

 With one or two chro- 

 matin granules associated 

 with a nutritive vacuole. 



Fungi. 



In the simplest case the cell of Beggiatoa contains only 

 cytoplasm without, so far as I can see by careful examin- 

 ation with the highest powers available, any differentiation 

 of chromatin grains or structures of a like nature. Neither 

 do I think that we can regard the protoplast as represent- 

 ing a nucleus. As Fischer points out, the idea that the 

 protoplast of the bacteria stains like a nucleus is not 

 correct, and, as I have been able to show, it certainly 

 does not give a phosphorus reaction like a nucleus. It 

 is, in fact, a simple undifferentiated mass of cytoplasm, 

 either homogeneous or at times exhibiting a foam struc- 

 ture. In this cytoplasm a few granules of chromatin may 

 become differentiated, and this is the first indication of 

 the separation of nuclear substance. Whether there are 

 any species of Beggiatoa or other bacteria which are 

 permanently without nuclear granules I do not know, and 

 it will be very difficult to prove it : but the fact that 

 under certain conditions these cells exist without them 

 seems to point to the conclusion that this may be the 

 primary cell structure, as has been surmised by Haeckel 

 and others. 



At an early stage in the evolutionary history of the 

 protoplasm, before a typical nucleus was evolved, we 

 appear to have had the development of colouring matter 

 for the function of assimilation, and a bifurcation into 

 the two distinct lines of descent of the fungi and the 

 algae. This appears to have been accompanied by two 

 distinct lines of nuclear evolution leading respectively to 

 the development of the central body of the Cyanophyceae 

 and the nuclear apparatus of the yeast plant. The possible 

 lines of development of the nucleus up to the yeast fungi 

 on the one side and to the Cyanophycea on the other are 

 clearly indicated in the diagram ; but between the yeast 

 fungi and the true fungi, and between the Cyanophycea; 

 and the Algae, there are gaps which we cannot bridge at 

 present. It is possible that the evolution of the typical 

 nucleus may have been brought about in the fungi by 

 the more definite association of the nuclear vacuole with 

 the homogeneous nuclear body, possibly accompanied by 

 a vacuolation of the latter, or that the nuclear body itself 



NO. 1873, ^OL. 72] 



