September 21, 1905] 



NA TURE 



525 



upon the fertilising tube, thus helping to bring the sexual 

 nuclei together. Stevens suggests that it may be of the 

 jiature of a dynamic centre, and he gave it the name 

 coenocenUum. It may be nutritive in function, and may 

 exert a chemotactic stimulus upon the sexual nuclei. 



It does not appear to be actually concerned in the fusion 

 of the sexual nuclei. In Peronospora parasitica, for 

 example, it completely disappears before the fusion of 

 the nuclei takes place. So far all the views as to its 

 function are purely hypothetical. It may be a mere 

 coincidence that it should become associated with the 

 sexual nuclei at the time they come together in the 

 obsphere. Its function may be totally unconnected with 

 these. From the fact that it stains so deeply in nuclear 

 stains, the substance of which it is composed may be 

 of the nature of nuclein, and it is possible that it may 

 be due to a substance secreted by the nuclei of the 

 oogonium for some special purpose connected with the 

 maturation of the oospore. It is possible that it may have 

 something to do with the formation of oil, which appears 

 in such abundance in the ripe oospores. It begins to dis- 

 appear just at the time the oil begins to form. 



It seems more likely that the function of the coeno- 

 centrum is connected with those metabolic activities of 

 the zygote, which must at this stage in its development 

 be very considerable, than with the exertion of an attractive 

 influence upon the sexual nuclei. It is difficult to see 

 how such a selective chemotactic stimulus could be exerted 

 as to act upon one nucleus only out of the large number 

 in the oogonium. But the evidence before us does not 

 admit of any definite solution of the problem at present. 

 The subject demands further investigation of such a kind 

 that a comparative study of the formation and disappear- 

 ance of the coenocentrum, the formation of the oil reserves, 

 and the changes in the nuclei, should be carried on side 

 bv side. 



The Nuclei of the Lower Plants. 



The presence of nuclei in the algae and fungi had already 

 been recorded by Nageli and many other observers shortly 

 after the discovery of the nucleus by Robert Brown, but 

 it is doubtful whether all the structures described as nuclei 

 by these early observers were really so. It is only in 

 comparatively recent times that it has been possible to 

 determine w'ith any degree of certainty that the minute 

 deeply stainable bodies described more especially by 

 Schmitz (1S79) could be regarded as nuclei. This deter- 

 mination was easily made for many of the algee, especially 

 by the researches of Strasburger, who described both the 

 structure and mode of division. But among the fungi the 

 structure and mode of division of the nuclei were prac- 

 tically unknown twenty years ago, and we have the 

 opinion expressed by De Bary in 1887 that the satisfactory 

 discrimination of true nuclei from other small bodies con- 

 tained in the protoplasm can only be obtained after renewed 

 investigation. 



Previous to 1887 cases of karyokinetic division in fungi 

 had been described bv Sadebeck (1883), Strasburger (1884), 

 Fisch (1885), and Eidam (1887). Hartog (1889) described 

 a process akin to karyokinesis in the Saprolegnieae, and 

 at the end of that year a true process of karyokinesis was 

 shown to occur in Peronospora. Since that time our 

 knowledge of the process of nuclear division in the fungi 

 has been largely extended, and the phenomenon has now 

 been found to be of general occurrence in the group, and 

 many of the forms are unusually favourable objects for 

 the studv of the process. 



The only groups of plants in which true nuclei have 

 not been found are, so far as I know, the bacteria, Cyano- 

 phyceae, and the yeast fungi. In the yeast plant there 

 is a large homogeneous spherical body which gives the 

 reactions of chromatin similar to the chromatin of true 

 nuclei. With this is associated a prominent vacuole which 

 contains a more or less amorphous substance of a 

 chromatin nature. The two appear to be very closely 

 related and undergo division simultaneously. 



The Cell Structure of the Cyanophyceae. 



It is easy to demonstrate in the living cell of the 



Cyanophyceae that the contents are differentiated into 



.two distinct regions : (i) an outer layer containing the 



•colouring matter ; and (2) a central colourless portion 



NO. 1873, VOL. 72] 



which is known as the central body. The central body 

 is considered by many investigators to be a true nucleus. 

 It contains a deeply staining granular substance which 

 to some extent resists the action of digestive fluids, and 

 is therefore similar to the chromatin in the nuclei of the 

 higher plants. In 1887 Scott was able to demonstrate 

 a reticulate structure in this body, and also saw some 

 indications during its division of a process akin to 

 karyokinesis. Zacharias also in the same year, largely 

 on micro-chemical grounds, concluded that it was a 

 nucleus. The problem has been the subject of investigation 

 by numerous observers since that date with very varying 

 results. These results may be shortly summarised as 

 follows : — The central body is not a nucleus (Macallum, 

 Fischer, Massart, Chodat). It is a nucleus of a simple 

 or rudimentary type (Hieronymus, Nadson, Butschli). It 

 is a true nucleus similar to that found in the higher plants, 

 and forms both chromosomes and spindle (Hegler, Kohl, 

 Olive, Phillips). 



The facts of the structure of this body, so far as I have 

 been able to ascertain them by the examination of the 

 cell both in the living and fixed conditions, are that it 

 possesses a vacuolate structure, associated with granules 

 which stain deeply in nuclear stains, resist the action 

 of digestive fluids, give a strong reaction for phosphorus 

 and masked iron, and, further, according to the recent 

 researches of Macallum, do not contain potassium. These 

 qualities are characteristic of nuclein, and there can be, 

 I think, no reasonable doubt that these granules are 

 comparable to the chromatin of a true nucleus. 



From a consideration of the facts we at present know 

 concerning the central body we cannot, I think, escape 

 the conclusion that it is of the nature of a nucleus, but 

 one of a simple or rudimentary type. It is not sharply 

 delimited from the surrounding cytoplasm, although it 

 sometimes appears as a vacuolar cavity in the centre of 

 the cell, with a vacuolar membrane around it. It seems 

 to me that we might very well regard it simply as a 

 specialised region of the cytoplasm which possesses a pro- 

 nounced vacuolation associated with granules of chromatin 

 or with a chromatin network. 



The Function of the Nucleus of the Cyanophyceae. 



The nucleus of the Cyanophyceae is very large, much 

 larger proportionally than the nuclei of the higher algae. 

 It gives also a proportionally stronger reaction for phos- 

 phorus. Some observers have considered the large size 

 and prominence of the central body as an argument against 

 its nuclear nature. In the algae the nuclei are much 

 smaller in proportion to the cell, and in many forms are 

 very difficult to make out. On the other hand the 

 pyrenoids which are present in the cells of Algs stain 

 more deeply in the nuclear stains, and give a much stronger 

 reaction for phosphorus than the nuclei. In Prassiola 

 parietina the pyrenoid is in the centre of the cell, and 

 both in the living condition and in stained preparations 

 is much more prominent than the slightly stained nucleus 

 on one side of it. So, also, in Zygnema there are two 

 star-shaped chromatophores, each with a large pyrenoid 

 in the middle, and between them a small very incon- 

 spicuous nucleus. 



My view is that the large size of the central body in 

 the Cyanophyceae may be connected with the development 

 of the chlorophyll assimilation ; that it may be held to 

 function both as a pyrenoid as well as a nucleus, and that 

 this receives support from what is observed in the coloured 

 bacteria, in which the cytoplasm contains a more abundant 

 supply of chromatin granules than do the colourless 

 bacteria. 



Structure of the Bacterial CeU. 



Owing to the small size of the bacterial cells it is very 

 difficult to arrive at a correct interpretation of the struc- 

 tures observed. The examination of the larger forms, 

 such as the various species of Beggiatoa, Chromatium, 

 Bacillus antkracis, Bacillus subtilis, &c., has, however, 

 revealed a certain differentiation, which enables us to come 

 to some conclusions as to their actual structure. Ernst 

 has shown that the contents of these cells are not homo- 

 geneous, as was formerly thought to be the case, but 

 show a differentiation into a less stainable substance, and 

 embedded in it one or more deeply stained granules. 



