September 6, 1900] 



NATURE 



443 



undergoing pathological changes. Corroborative observations 

 on endogenous formation were also given by his brother, Harry 

 Goodsir, in 1845. These observations on the part which the 

 nucleus plays by cleavage in the formation of young cells by 

 endogenous development from a parent centre — that an organic 

 continuity existed between a mother cell and its descendants 

 through the nucleus — constituted a great step in advance of the 

 views entertained by Schleiden and Schwann, and showed that 

 Barry and the Goodsirs had a deeper insight into the nature 

 and functions of cells than was possessed by most of their con- 

 temporaries, and are of the highest importance when viewed in 

 the light of recent observations. 



In 1 84 1 Robert Remak published an account of the presence 

 of two nuclei in the blood corpuscles of the chick and the pig, 

 which he regarded as evidence of the production of new 

 corpuscles by division of the nucleus within a parent cell ; but 

 it was not until some years afterwards (1850 to 1855) that he 

 recorded additional observations and recognised that division of 

 the nucleus was the starting-point for the multiplication of cells 

 in the ovum and in the tissues generally. Remak's view was 

 that the process of cell-division began with the cleavage of the 

 nucleolus, followed by that of the nucleus, and that again by 

 cleavage of the body of the cell and of its membrane. KoUiker 

 had previously, in 1843, described the multiplication of nuclei 

 in the ova of parasitic worms, and drew the inference that in 

 the formation of young cells within the egg the nucleus under- 

 went cleavage, and that each of its divisions entered into the 

 formation ot a new cell. By these observations, and by others 

 subsequently made, it became obvious that the multiplication of 

 animal cells, either by division of the nucleus within the cell, or 

 by the budding off of a part of the protoplasm of the cell, was 

 to be regarded as a widely spread and probably a universal 

 process, and that each new cell arose from a parent cell. 



Pathological observers were, however, for the most part 

 inclined to consider free cell-formation in a blastema or exuda- 

 tion by an aggregation of molecules, in accordance with the 

 views of Henle, as a common phenomenon. This proposition 

 was attacked with great energy by Virchow in a series of 

 memoirs published in his " Archiv," commencing in vol. i. 

 1847, and finally received its death-blow in his published lec- 

 tures on Cellular Pathology, 1858. He maintained that in 

 pathological structures there was no instance of cell development 

 lie novo ; where a cell existed, there one must have been before. 

 Cell-formation was a continuous development by descent, which 

 he formulated in the expression oinnis cellula e celhtld. 



Karyokinesis. 



Whilst the descent of cells from pre-existing cells by division 

 of the nucleus during the development of the egg, in the embryos 

 of plants and animals, and in adult vegetable and animal tissues, 

 both in healthy and diseased conditions, had now become 

 generally recognised, the mechanism of the process by which 

 the cleavage of the nucleus took place was for a long time un- 

 known. The discovery had to be deferred until the optician 

 had been able to construct lenses of a higher penetrative power, 

 and the microscopist had learned the use of colouring agents 

 capable of dyeing the finest elements of the tissues. There was 

 reason to believe that in some cases a direct cleavage of the 

 nucleus, to be followed by a corresponding division of the cell 

 into two parts, did occur. In the period between 1870 and 1880 

 observations were made by Schneider, Strasburger, Btitschli, 

 Fol, van Beneden and Flemming, which showed that the divi- 

 sion of the nucleus and the cell was due to a series of very remark- 

 able changes, now known as indirect nuclear and cell division, 

 or karyokinesis. The changes within the nucleus are of so 

 complex a character that it is impossible to follow them in detail 

 without the use of appropriate illustrations. I shall have to con- 

 tent myself, therefore, with an elementary sketch of the process. 



I have previously stated that the nucleus in its passive or 

 resting stage contains a very delicate network of threads or fibres. 

 The first stage in the process of nuclear division consists in the 

 threads arranging themselves in loops and forming a compact 

 coil within the nucleus. The coil then becomes looser, the loops 

 of threads shorten and thicken, and somewhat later each looped 

 thread splits longitudinally into two portions. As the threads 

 stain when colouring agents are applied to them, they are called 

 chromatin fibres, and the loose coil is the chromosome(Waldeyer). 



As the process continues, the investing membrane of the 

 nucleus disappears, and the loops of threads arrange themselves 

 within the nucleus so that the closed ends of the loops are 



NO. 1 6 10. VOL. 62] 



directed to a common centre, from which the loops radiate out- 

 wards and produce a starlike figure (aster). At the same time 

 clusters of extremely delicate lines appear both in the nucleo- 

 plasm and in the body of the cell, named the achromatic figure, 

 which has a spindle-like form with two opposite poles, and stains 

 much more feebly than the chromatic fibres. The loops of the 

 chromatic star then arrange themselves in the equatorial plane of 

 the spindle, and bending round turn their closed ends towards 

 the periphery of the nucleus and the cell. 



The next stage marks an important step in the process of 

 division of the nucleus. The two longitudinal portions, into 

 which each looped thread had previously split, now separate 

 from each other, and whilst one part migrates to one pole of 

 the spindle, the other moves to the opposite pole, and the free 

 ends of each loop are directed towards its equator (metakinesis). 

 By this division of the chromatin fibres, and their separation 

 from each other to opposite poles of the spindle, two starlike 

 chromatin figures are produced (dyaster). 



Each group of fibres thickens, shortens, becomes surrounded 

 by a membrane, and forms a new or daughter nucleus (dispirem). 

 Two nuclei therefore have arisen within the cell by the divisiorv 

 of that which had previously existed, and the expression formu- 

 lated by Flemming— <;w«z.f nucleus e nucleo — is justified. 

 Whilst this stage is in course of being completed, the body of 

 the cell becomes constricted in the equatorial plane of the 

 spindle, and, as the constriction deepens, it separates into two 

 parts, each containing a daughter nucleus, so that two nucleated 

 cells have arisen out of a pre-existing cell. 



A repetition of the process in each of these cells leads to the 

 formation of other cells, and, although modifications in details 

 are found in different species of plants and animils, the multi- 

 plication of cells in the egg and in the tissues generally on 

 similar lines is now a thoroughly established fact in biological 

 science. 



In the study of karyokinesis, importance has been attached to 

 the number of chromosomes in the nucleus of the cell. Fiem- 

 ming had seen in the Salamander twenty-four chromosome fibres, 

 which seems to be a constant number in the cells of epithelium 

 and connective tissues. In other cells again, especially in the 

 ova of certain animals, the number is smaller, and fourteen, 

 twelve, four, and even two only have been described. The 

 theory formulated by Eoveri that the number of chromosomes is 

 constant for each species, and that in the karyokinetic figures 

 corresponding numbers are found in homologous cells, seems to 

 be not improbable. 



In the preceding description I have incidentally referred to 

 the appearance in the proliferating cell of an achromatic spindle- 

 like figure. Although this was recognised by Fol in 1873, it is 

 only during the last ten or twelve years that attention has been 

 paid to its more minute arrangements and possible signification 

 in cell-division. 



The pole at each end of the spindle lies in the cell plasm 

 which surrounds the nucleus. In the centre of each pole is a 

 somewhat opaque spot (central body) surrounded by a clear 

 space, which, along with the spot, constitutes the centrosome or 

 the sphere of attraction. From each centrosome extremely 

 delicate lines may be seen to radiate in two directions. One set 

 extends towards the pole at the opposite end of the spindle, and, 

 meeting or coming into close proximity with radiations from it, 

 constitutes the body of the spindle, which, like a perforated 

 mantle, forms an imperfect envelope around the nucleus during 

 the process of division. The other set of radiations is called the 

 polar, and extends in the region of the pole towards the periphery 

 of the cell. 



The question has been much discussed whether any con- 

 stituent part of the achromatic figure, or the entire figure, exists 

 in the cell as a permanent structure in its resting phase ; or if it 

 is only present during the process of karyokinesis. During the 

 development of the egg the formation of young cells, by division 

 of the segmentation nucleus, is so rapid and continuous that the 

 achromatic figure, with the centrosome in the pole of the spindle, 

 is a readily recognisable object in each cell. The polar and 

 spindle-like radiations are in evidence during karyokinesis, and 

 have apparently a temporary endurance and function. On the 

 other hand, van Beneden and Boveri were of opinion that the 

 central body of the centrosome did not disappear when the 

 division of the nucleus came to an end, but that it remained as 

 a constituent part of a cell lying in the cell plasm near to the 

 nucleus. Flemming has seen the central body with its sphere 

 in leucocytes, as well as in epithelial cells and those of othe 



