May 19, 1898] 



NATURE 



63 



THE PRESENT POSITION OF SOME CELL 



PROBLEMS. 

 T~^URIXG the last two decades or so a new branch of science 

 has been quietly, but rapidly, working its way from a 

 position of comparative obscurity to one of considerable im- 

 portance. This new-comer has been designated Cytology, and 

 it embraces as its province that department of knowledge which 

 centres around the cell, whether this body be regarded from its 

 structural or from its functional aspect. And cytology, which 

 is still a young offshoot both from botany and zoology, pos- 

 sesses one strongly marked advantage, viz. that of providing 

 a common ground on which the botanist and the zoologist may 

 still meet to di-cuss questions of equal interest to each. For in 

 dealing with the cell we are approaching facts and phenomena 

 which are essentially shared or exhibited by animals and plants 

 alike, and, indeed, the measure of their relative importance can 

 be gauged by the degree in which they reappear in each of the 

 two great divisions of organic life ; although in most other 

 respects the animals and plants have followed widely diverging 

 paths of development. 



The cell was long ago recogni-sed as the structural unit of an 

 organism, but the relations of its various parts to one another 

 were overlooked or misunderstood, and we are still far from 

 arriving at a satisfactory solution of the difficulties which each 

 investigator meets when attacking the problems presented by 

 any special case; nevertheless, some general facts have been dis- 

 covered which serve as landmarks to guide future exploration. 



In all but the very lowest forms of life, and in some others 

 which are probably degenerate, we recognise clearly enough 

 that the protoplasm of an organism contains one or more nuclei 

 within its substance. Commonly, though by no means in- 

 variably, each nucleus is associated with a definite ma.ss of pro- 

 toplasm which is segregated, more or less strictly, from the rest 

 by means of membranous partitions. These partitions are not, 

 however, necessarily always present. Some animals, and many 

 of the lower plants, possess a protoplasm in which are distributed 

 large numbers of nuclei, which thus appear to lie embedded in a 

 common matrix. Instances of this are seen in Vaucheria and in 

 the embryonic stages of Peripattis. But although the nuclei are 

 thus scattered, there is a considerable body of evidence to show 

 that their respective spheres of influence are tolerably clearly 

 defined, just as are those of different countries, even when these 

 are not delimited by obvious boundaries like rivers or mountain 

 ranges. 



On the other hand, just as there are roads and traffic between 

 two neighbouring countries, so it has been shown by several 

 observers that even where the "cells" are separated by walls 

 from each other, the adjacent protoplasms are often connected 

 by fine threads of the living substance which traverse the inter- 

 vening cell walls. The phenomenon seems to have been occa- 

 sionally seen without apparently its importance being realised, 

 but Tangl clearly demonstrated it for plant cells (Endosperm) 

 almost twenty years ago. Since that time the investigations of 

 Gardiner, Kienitz-Gerloff, and others have shown that what were 

 once thought to be merely isolated cases may possibly turn out 

 to form rather the rule than the exception. There can be but 

 little doubt that the improved uranium-osmium method of 

 Kolossow, which has recently been employed with considerable 

 succe.ss by Gardiner, will materially extend our knowledge in 

 this direction, and will confirm what most of us have for a long 

 time held, that the difference between such a plant as Canlerpa 

 and the ordinary multicellular forms is rather one of degree, the 

 result of specialisation, than one of kind. Thus during the 

 germination of some alga;, certain of the Fncacea: for example, 

 the embryo exists for a considerable time in a multinucleate con- 

 dition, the cell walls only appearing at a later stage. The same 

 is also seen during the development of the endosperm in a flower- 

 ing plant, and still more .strikingly during the germination of 

 the spore of Isoetes or of Selaginella. The occurrence of a stage 

 in the development of many plant tissues, during which the con- 

 stituent cells are sliding past each other in adjacent rows, is 

 seen to furnish no real argument for a protoplasmic discontinuity 

 at this period, when it is remembered that not only are the walls 

 still soft, but that they actually contain a nitrogenous body 

 which is almost certainly protoplasm in their substance. On the 

 animal side also evidence is not lacking to show that in some of 

 the higher forms, at least in the earlier stages, protoplasmic 

 continuity is of frequent occurrence ; arid it also obtains, accord- 

 ing to Schuberg and others, between the cells of some tissues 

 in the adult animal. 



NO. 1490, VOL. 58] 



Nevertheless, the want of such a continuity in nerves, e.g. in 

 the ganglionic cells, suffices to show that it is unsafe lo generalise 

 on a priori grounds too freely, for it is in nerves, perhaps more 

 than in most other tissues, that a direct continuity might have 

 been expected. And it is the more necessary to emphasise the 

 lesson derived from a study of the histology of nervous tissues, 

 inasmuch as a continuity of protoplasm has been generally 

 assumed to exist in the tissues of motile organs of plants, on 

 purely physiological grounds, although it may not have been 

 demonstrated histologically. 



The r6le played by the nucleus in influencing or in deter- 

 mining the mode of special activity manifested by its attendant 

 protoplasm is one of great interest, and a great deal of light has 

 been thrown upon it within recent years. Haberlandt and others 

 have clearly shown that in cases where metabolism was more 

 active in one region of the cell than in another, the nucleus 

 commonly migrates to this locality. Beautiful examples of this 

 may be observed during the thickening of the walls so frequently 

 met with in the protective layer of seeds or fruits. Thus if the 

 development of the seed of the common night-shade {Solanttm 

 Dulcamara) be followed, it will be seen that in the young stages 

 of the large cells which ultimate'y give rise to the hard shell of the 

 seed, the nucleus occupies a central position. Later on, the 

 nucleus becomes lodged in close proximity to the inner wall of 

 the cell, and this then begins to thicken. This deposition of 

 thickening substances spreads to the lower (or inner) parts of the 

 lateral walls ; whilst their outer portions, as well as the whole 

 of the external wall, which is remote from the nucleus, remains 

 thin. Again, it has been observed by Istvanffi that when the 

 hypha of a fungus is about to branch, the nucleus is discoverable 

 at a spot just beneath which the outgrowth is about to arise. 



The well-known and highly characteristic appearance of the 

 large nuclei met with in tissues the cells of which are in an 

 active state of division, is all evidence of the important in- 

 fluence of these bodies over the process. So also is the fact 

 that those cells which are the last to lose the faculty of re- 

 suming an embryonic condition {i.e. of giving rise to fresh 

 tissues) retain these nuclear peculiarities longest. This point is 

 well brought out in a study of the cells of a growing root, for 

 it is easily seen that those which form the layer known as the 

 pericycle keep the primitive appearance of their nuclei the 

 longest, and it is in this layer that the new structures, the 

 lateral roots, when they occur do actually originate. Again* 

 when new structures are about to be formed from tissues already 

 adult, or even senescent, the first obvious sign of the new im- 

 pulse is detected in a change in the nuclei of the cells, a change 

 which depends as much on chemical as on physical differences. 

 In cells which are secreting, whether belonging to animals or to 

 plants, the nuclei are observed to pass through a remarkable 

 .series of changes, which may even result in the temporary 

 differentiation of the peculiar so-called chromatic elements, 

 resembling if indeed not identical with those appearing during 

 nuclear division. Much the same is to be seen in the huge 

 nuclei often present in the " foot cells " in an animal testis, 

 around which the young immature spermatozoids cluster in 

 groups, apparently deriving from the chemical activity of these 

 cells the nourishment requisite for the completion of their 

 development. 



Even more conclusive evidence as to the close relation 

 between the metabolism of the external protoplasm (conveniently 

 distinguished as cytoplasm) and the nucleus is furnished by the 

 different behaviour of nucleated and non-nucleated fragments of 

 protoplasm respectively. It is quite possible, by taking appro- 

 priate measures, to vivisect a single cell, so that one portion 

 shall contain a nucleus and the other not. The former half 

 commonly regenerates itself, and if derived from a plant cell, 

 forms around itself a new cell wall ; on the other hand, the 

 non-nucleated fragment sooner or later perishes, although it may 

 continue for a time to exhibit normal vital functions. Usually, 

 however, it is able neither to secrete on its surface a membrane, 

 nor to engage on constructive metabolism. 



But interesting and suggestive as are the relations which can 

 be discerned between the cytoplasm and the nuclei of cells in a 

 condition of comparative repose, they are almost eclipsed by the 

 wonderful series of changes which recur with Surprising uni- 

 formity each time the nucleus and the cell divides. Nor is it 

 always easy correctly to estimate the relative importance of the 

 various structural elements which are involved or concerned in 

 the process. 



Of late years we have heard a great deal about a minute 



