divisions, and, as we shall see later, has also undergone internal changes. In this state it is 'ripe,' that is, it is ready to enter into conjugation with the nucleus of a male cell, and this we have already recognized as the essential element in the process of fertilization. These processes of 'maturation of the ovum' are common to all animal ova which require fertilization, and they follow almost the same course, only that in many cases the second division of the first polar body does not take place, so that only two polar bodies in all are formed. All these processes have nothing directly to do with fertilization, but it is only through them that the ovum becomes capable of fertilization. This does not prevent the spermatozoon from previously making its way into the ovum, for this is usually the case (Fig. 75, _A_, _sp_); there it waits until the second 'directive division' of the ovum has been accomplished, utilizing the time to become transformed in the manner necessary for the conjugation of the two nuclei. Only in a few species, for example in the sea-urchin, does the egg complete its polar divisions within the ovary, therefore before it has come into contact with the sperm at all. [Illustration: FIG. 75. Process of fertilization in _Ascaris megalocephala_, the thread-worm of the horse, adapted from Boveri and Van Beneden. _A_, ovum in process of the first directive division; _Rk_ 1, first polar body; _sp_, spermatozoon with two chromosomes in its nucleus, attaching itself to the ovum, and about to penetrate into it; a protrusion of the egg-protoplasm is meeting it. _B_, the second directive division has been completed; _Rk2_, the second polar body; _Eik_, the reduced nucleus of the ovum. The first polar body (_Rk_ 1) has divided into two daughter-cells, _spk_; the nucleus of the spermatozoon remains visible with its two centrospheres (_csph_). _C_, the sperm nucleus (♂_k_) and the ovum nucleus (♀_k_) have grown, each has two loop-like chromosomes; only the male nucleus has a centrosphere, which has already divided into two (_csph_). _D_, the two nuclei lie apposed between the poles of the nuclear spindle. _E_, the four chromosomes have split longitudinally; the spindle for the first division of the ovum (the segmentation spindle, _fsp_) has been formed. _F_, divergence of the daughter-chromosomes towards the two poles; division of the ovum into the first two cleavage cells or embryonic cells.]