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. [Illustration: 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_]. [Illustration: FIG. 38.--Formation and conjugation of the pronuclei in _Ascaris megalocephala_. (From _Quain's Anatomy_, after E. von Beneden.) _f_, female pronucleus; _m_, male pronucleus; _p_, one of the polar bodies. I. The second polar body has just been extruded; both male and female pronuclei contain two chromatin particles; those of the male pronucleus are becoming transformed into a skein. II. The chromatin in both pronuclei now forms into a skein. II _a_. The skeins are more distinct. Two attraction (or protoplasmic) spheres, each with a central particle united with a small spindle of achromatic fibres, have made their appearance in the general substance of the egg close to the mutually approaching pronuclei. The male pronucleus has the remains of the body of the spermatozoön adhering to it.