xxii GENERAL INTRODUCTION. 



rounded, but capable sometimes of changes of shape. It consists prin- 

 cipally of a substance termed nuclein from a chemical point of view, or 

 chromatin from its marked physical peculiarity, that of readily absorbing, 

 and to a much greater degree than the rest of the cell, various kinds of 

 stains such as carmine. But this simple structure is rarely retained. 

 The nucleus is limited externally by a nuclear membrane, its chromatin 

 is disposed in very various ways, as a reticulum, a much coiled thread, 

 in fragments as a lining to the nuclear membrane and one or more 

 central spots. To nodal thickenings of the thread, &c., or to the 

 fragments of chromatin, the term nucleolus is generally applied. The 

 intervals between the chromatin elements are occupied by a nuclear 

 fluid, composed of a protoplasm (caryoplasma), which may be resolved 

 like the protoplasm of the cell (of which it is a part) into a denser and 

 more fluid portion. As this protoplasm takes up stains but slightly, 

 it is frequently designated achromatin. The structure of the nucleus is 

 not always the same at all periods of its life ; it may be at first homo- 

 geneous, but as a rule assumes one of the more complex forms. 



The process of division of the cell may be comparatively simple or 

 complicated. The division of the protoplasm is preceded, or accompanied, 

 by division of the nucleus. The latter process may be direct or amitotic, 

 the nucleus simply elongating, and being split by a constriction. Or it 

 may be indirect or mitotic, the achromatin being disposed in lines parallel 

 to the long axis of the nucleus, making the figure of a spindle, and the 

 chromatin grouped at the centre of the spindle, dividing into parts which 

 move in opposite directions to either pole of the spindle, whilst a con- 

 striction splits the nucleus in two. It is rare for the chromatin to be 

 grouped in two masses on the equator and the split of the nucleus to 

 take place through its poles. The figures seen in the process are spoken 

 of as karyokinetic. It has been found that the typical mitotic and 

 amitotic modes of divisions are connected by intermediate phases, at 

 least in some tissue cells. The nuclear membrane is dissolved in mitosis 

 and reconstituted round the new nuclei 1 . 



1 The denser mitome of the nucleus and the body of the cell may give rise to an equatorial 

 plate, or the former may do so, and not the latter. This plate, which is common in plants, but has 

 only been detected in certain tissue-cells of Arthropoda, may evanesce, or fission may take place 

 through its median plane. It may be noted that the mitome of the cell-body is frequently arranged 

 in radii during the nuclear changes, and that a clear spot, the polar spot or corpuscle, may appear at 

 each pole of the spindle. For the structure of the cell, see Carnoy, ' La Biologic cellulaire,' Lierre, 

 Fasc. i. pt. 2, 1884 ; for cell-division (cytodieresis) in Arthropoda, Id. ' La Cellule/ Lierre, i. 1885-6, 

 and the summary of both papers by A. Bolles Lee in Q. J. M. xxvi. 1886, p. 481 ; cf. also Flemming 

 and Carnoy in Z. A. ix. 1886. For the fission of the giant-cells, &c. in the medulla of bone, see 

 Denys, < La Cellule,' ii. (2), 1887. 



