1 8 MORPHOLOGY OF THE CELL. 



must be given of the structure of the nucleus in a state of rest'. The nucleus consists 

 of a ground-substance which is homogenous, but sometimes presents, after careful pre- 

 paration and staining, a finely punctuated appearance : it is denser towards the periphery, 

 and thus constitutes a sort of membrane which gives to the nucleus its sharply-defined 

 contour. In the ground-substance a more solid substance is present either in the form 

 of granules, the larger of which are the nucleoli, or in the form of fibrillae united into 

 a network, the points of junction of the fibrillae, the nodes, being somewhat thickened. 

 Since, when the tissue is carefully stained with certain colouring-matters, the granules 

 or the network become coloured more readily than the ground-substance of the nucleus, 

 Flemming has designated the former chromatin and the latter achromatin. 



When nuclear division is about to take place the nucleus presents a coarsely 

 granular appearance ; the granules then coalesce to form convoluted threads, and these, 

 together with the fibrillae of the nuclear network which have become free, arrange 

 themselves parallel to the long axis of the now elongated nucleus. At this time the 

 nucleus has usually lost (Strasburger mentions Spirogyra nitida and the mother-cells of 

 the spores of Equisetum as exceptions) its well-defined contour: it has assumed a 

 spindle-shape, and an aggregation of protoplasm has been formed at each pole, in which 

 a radial arrangement of the granules, with the pole as a centre, is usually evident. The 

 fibrillae of chromatin now contract so as to form a disc of greater or less thickness 

 {equatorial plate of Flemming, nuclear disc of Strasburger) in the equatorial plane of the 

 spindle, and faint striae can now be detected between the nuclear disc and each pole, 

 some of which apparently pass through the disc from one pole to the other. The 

 nuclear disc now splits into two halves lying side by side, and each half travels to the 

 corresponding pole along the fibrillae which cause the striation of the spindle, and each 

 then constitutes a new nucleus at the pole. The few fibrillae which now connect the 

 two nuclei are augmented by the formation of new ones, and the whole complex of 

 fibrillae assumes the form of a biconvex lens and extends across the cell. A row" of 

 granules now makes its appearance in the equatorial plane of the fibrillae consisting, 

 according to Strasburger, of starch or of a substance allied to starch and cellulose ^ ; 

 this is the cell-plate, and in this the cell-wall is simultaneously formed. If the complex 

 of fibrillae does not extend across the cell, so as to be in contact with both its lateral walls, 

 the new wall is formed in contact with one lateral wall, and the complex gradually grows 

 towards the other wall by the formation of new fibrillae in which additions to the new cell- 

 wall are deposited. The new wall is clothed on each side with a layer of protoplasm, the 

 nucleus assumes its normal position in the cells, and the process of division is complete. 



The above description applies to the most complicated form of karyokinesis, as it 

 occurs in relation to the development of the reproductive cells and to the reproductive 

 processes of the higher plants. In the cell-division of the vegetative organs of these 

 plants the process is usually simpler ; thus, according to Schmitz, in the meristematic 

 cells of Phanerogams the nucleus becomes elongated, the ends being swollen and the 

 middle connecting part remaining narrow and presenting a longitudinal striation ; the 

 swollen ends become defined as new nuclei, and a cell-plate appears in the narrow 

 portion in which the cell-wall is soon formed ; the narrow portion gradually loses its 

 striated appearance and assumes the appearance of the cell-protoplasm. In the cell- 

 division of Spirogyra, as we have already seen, no cell-plate is formed. Strasburger 

 draws attention to the fact that in those Thallophytes (excluding the Characeae) in which 

 karyokinesis has been observed, the cell-plate is not formed, as in the higher plants, in a 

 complex of fibrillae which connect the two new nuclei, but in a bridge of protoplasm 

 which connects the two nuclei and extends across the cell in the plane of division.] 



^ [This account is taken from the above-quoted work of Strasburger, and from those of Schmitz 

 (Sitzber. d. niederrhein. Ges. in Bonn, 1880) and Flemming (Arch. f. micr. Anat. vol. 18).] 



* [According to Schmitz {loc. cii.) the granules in question are microso^nata, i.e. constituent 

 elements of the protoplasm.] 



