Fertilization having been thus effected by fusion of the male and female pronuclei into a single (or new) nucleus, this latter body proceeds to exhibit complicated processes of karyokinesis, which, as before shown, are preliminary to nuclear division in the case of egg-cells. Indeed the karyokinetic process may begin in both the pronuclei before their junction is effected; and, even when their junction is effected, it does not appear that complete fusion of the so-called chromatin elements of the two pronuclei takes place. For the purpose of explaining what this means, and still more for the purpose of giving a general idea of the karyokinetic processes as a whole, I will quote the following description of them, because, for terseness combined with lucidity, it is unsurpassable.
Fig. 36.—Karyokinesis of a typical tissue-cell (epithelium of Salamander). (After Flemming and Klein.) The series from A to I represents the successive stages in the movement of the chromatin fibres during division, excepting G, which represents the “nucleus-spindle” of an egg-cell. A, resting nucleus; D, wreath-form; E, single star, the loops of the wreath being broken; F, separation of the star into two groups of U-shaped fibres; H, diaster or double star; I, completion of the cell-division and formation of two resting nuclei. In G the chromatin fibres are marked a, and correspond to the “equatorial plate"; b, achromatin fibres forming the nucleus-spindle; c, granules of the cell-protoplasm forming a “polar star.” Such a polar star is seen at each end of the nucleus-spindle, and is not to be confused with the diaster H, the two ends of which are composed of chromatin.
Researches, chiefly due to Flemming, have shown that the nucleus in very many tissues of higher plants and animals consists of a capsule containing a plasma of “achromatin,” not deeply stained by re-agents, ramifying in which is a reticulum of “chromatin” consisting of fibres which readily take a deep stain. (Fig. 36, A). Further it is demonstrated that, when the cell is about to divide into two, definite and very remarkable movements take place in the nucleus, resulting in the disappearance of the capsule and in the arrangement of its fibres first in the form of a wreath (D), and subsequently (by the breaking of the loops formed by the fibres) in the form of a star (E). A further movement within the nucleus leads to an arrangement of the broken loops in two groups (F), the position of the open ends of the broken loops being reversed as compared with what previously obtained. Now the two groups diverge, and in many cases a striated appearance of the achromatin substance between the two groups of chromatin loops is observable (H). In some cases (especially egg-cells) this striated arrangement of the achromatin is then termed a “nucleus-spindle,” and the group of chromatin loops (G, a) is known as “the equatorial plate.” At each end of the nucleus-spindle in these cases there is often seen a star consisting of granules belonging to the general protoplasm of the cell (G, c). These are known as “polar stars.” After the separation of the two sets of loops (H) the protoplasm of the general substance of the cell becomes constricted, and division occurs, so as to include a group of chromatin loops in each of the two fission products. Each of these then rearranges itself together with the associated chromatin into a nucleus such as was present in the mother cell to commence with (I)[13].
Since the above was published, however, further progress has been made. In particular it has been found that the chromatin fibres pass from phase D to phase F by a process of longitudinal splitting (Fig. 37 g, h; Fig. 38, VI, VII)—which is a point of great importance for Weismann’s theory of heredity,—and that the protoplasm outside the nucleus seems to take as important a part in the karyokinetic process as does the nuclear substance. For the so-called “attraction-spheres” (Fig. 38 II a, III, III a, VIII to XII), which were at first supposed to be of subordinate importance in the process as a whole, are now known to take an exceedingly active part in it (see especially IX to XI). Lastly, it may be added that there is a growing consensus of authoritative opinion, that the chromatin fibres are the seats of the material of heredity, or, in other words, that they contain those essential elements of the cell which endow the daughter-cells with their distinctive characters. Therefore, where the parent-cell is an ovum, it follows from this view that all hereditary qualities of the future organism are potentially present in the ultra-microscopical structure of the chromatin fibres.
Fig. 37.—Study of successive changes taking place in the nucleus of an epithelium cell, preparatory to division of the cell. (From Quain’s Anatomy, after Flemming.) a, resting cell, showing the nuclear network; b, first stage of division, the chromatoplasm transformed into a skein of closely contorted filaments; c to f, further stages in the growth and looping arrangement of the filaments; g, stellate phase, or aster; h, completion of the splitting of the filaments, already begun in f and g; i, j, k, successive stages in separation of the filaments into two groups; l, the final result of this (diaster); m to q, stages in the division of the whole cell into two, showing increasing contortion of the filaments, until they reach the resting stage at q.
