Page:1902 Encyclopædia Britannica - Volume 27 - CHI-ELD.pdf/372

 0 Y T O LOGY plants are concerned, such transverse divisions do not take times granular, and in other cases it is a homogeneous body place. In the process of division of a spore mother-cell the with no visible structure or differentiation. The special spirem thread is divided longitudinally before the segmen- function of this organ has been a source of controversy tation occurs (Fig. 1, B), and this is preceded by a peculiar during the past few years, and much uncertainty still exists contraction of the thread around the nucleolus which has as to its true nature. There does not appear to be any been termed synapsis (Fig. 1, A). It has been suggested staining reaction which absolutely separates it from other that this may be connected with the early splitting of structures in the cell. In resting nuclei it often stains more the thread, but some observers regard it as a contraction deeply than any other of the nuclear constituents. In due to reagents. The segments of each chromosome are nuclei in a dividing stage it tends to disappear or break up usually twisted upon each other and may be much contorted into granules, and always stains less intensely than in the (Fig. 1, C, D). Soon after the segmentation the chromo- resting stage. So far as we know at present, there are somes begin to shorten and thicken, and another peculiar two distinct types of nucleoli in plants, one of which phenomenon is then observed—each longitudinal half of stains exactly like chromatin in the chromosome stage, the chromosome exhibits a double row of dots instead of a the other more like the cytoplasm. Many observers are single row, probably due to a second division of the inclined to the view that true nucleoli do not contain chromatin granules. This is not, however, accompanied chromatin, but our knowledge of their behaviour towards by a longitudinal fission of the linin segment, and the the various reagents which are stated to differentiate granules are soon obscured by the continued shortening chromatin is not sufficiently definite to allow that view to and thickening of the chromosome, which finally becomes pass uncriticized. Nor is our knowledge of the morphohomogeneous (Fig. 1, E). The longitudinal segments logical behaviour of the nucleolus sufficient to enable us of the chromosome become more or less fused together, to draw a definite distinction between “ true nucleoli ” and especially at the ends, by which variously shaped figures nucleolus-like bodies. It may be that all these structures are produced of greater or less regularity (Fig. 1, E). The should be regarded as true nucleoli of a different chemical chromosomes now become attached to the spindle-fibres nature (Montgomery). The nucleolus is most commonly (Fig. 1, F, G), and as the daughter chromosomes become considered to be a store of reserve material for the use pulled asunder they often appear more or less V-shaped, of the nucleus during division, serving also in part to so that each pair appears as a closed ring of irregular nourish the chromosomes. This view, which was formerly shape, the ends of the V’s being in contact thus—O held by Strasburger, is supported by the behaviour of the (Fig. 1, H, I, J, K.). This V has been variously inter- nucleolus during division. In the resting condition it preted. Some observers consider that it is due to bending, stains more deeply than the chromatin network, but as the others to the early longitudinal splitting, of the daughter chromosomes are formed it loses its capacity for stains to chromosomes, the segments of which remaining connected a large extent, and the staining capacity of the chromosomes together at one end, produce a V-shaped figure. From the at the same time increases; in some cases the nucleolus recent observations of Strasburger it appears probable breaks up, and particles of nucleolar substance are often that the latter interpretation is the correct one in most found attached to the chromosomes, other particles being cases, and that the former only occurs exceptionally. The at the same time found in the cytoplasm. According to form of division which is thus characterized by the early Strasburger’s more recent explanation, the nucleolus is longitudinal fission of the chromosomes and the formation concerned mainly in the formation of the nuclear spindle. of more or less regular rings is called the heterotype division. As it dissolves, it is taken up by the kinoplasm, which is In the one case, therefore, this ring is formed by a single thereby stimulated to form the spindle-figure, and the longitudinal fission and subsequent bending of the two maximum development of the spindle fibres coincides with halves of the chromosome; in the other, by two longitudinal the complete or partial disappearance of the nucleolus. fissions at right angles to one another. In either case the Another explanation given, which is probably the best, is first division of the germ-nucleus in which the reduced that the nucleolus is a store of reserve material both for number of chromosomes appears, results in an equal division the chromosomes and for the production of the spindle. The centrosome is a minute homogeneous granule found of the chromatin granules or ids to each daughter nucleus, just as in the vegetative nuclei. But now comes the im- in the cytoplasm of some cells in the neighbourhood of the portant question. Is the division which next takes place nucleus. It is generally surrounded by a clear Centr0m in the daughter nuclei of this nature, or is it effected in space, which is in turn surrounded by a granular some such a manner as to produce a qualitative separation of the or radiating cytoplasmic substance, the whole chromatin % In other words, is the division of the chromo- being known, as the centrosphere or attraction-sphere. somes of the daughter nuclei longtitudinal or transverse I According to the researches of Van Beneden, Boveri, and Some observers contend that where the V shape is pro- others, it is an important special organ of the animal cell, duced by a bending of the chromosome, the division and has been regarded as a regular constituent of it, like is accomplished by a separation of the V’s at the apex, the nucleus. It is not so common in plant cells, and its and that in this way we get a true transverse or qualita- presence' has only been certainly demonstrated in the tive division. But from the observations of Farmer, Thallophytes and Bryophytes. In the higher plants the Sargant, Gregoire, Strasburger, and others, it appears that structures which have been often described as centrosomes in all such cases the division takes place by a longi- are too indefinite in their constitution to allow of this tudinal splitting of the V, and not by its separation at the interpretation being placed upon them. Even in the lower apex; and where the V’s are produced, not by bending, plants, while the centrospheres may be very apparent, the but by the early longitudinal splitting of the chromosomes, granules found in them are not sufficiently uniform either it is evident that their separation at the apex is simply the in number or general character to allow of their being completion of this longitudinal fission, and not a transverse regarded as centrosomes (Farmer), and many of the centrodivision at all (Fig. 1, L to P). somes described are probably nothing more than granules In the majority of plant-nuclei, both in the higher and of the fragmented nucleolus, or possibly extra-nuclear lower plants, there is found, in addition to the chromatin nucleoli. They do not appear to be permanent organs of cell. They are prominent during cell-division, but may Nucleolus ne^wor^:> a deeply stained spherical or slightly the ‘ irregular body (sometimes more than one) called disappear in the resting stage. They are more easily seen, the nucleolus (Fig. 1, A to D). It is often vacuolar, some- when the nucleus is about to undergo mitosis, at the ends

336