That we may be able to penetrate still more deeply into the processes of fertilization, the best illustration to take seems to me to be, as yet, the ovum of the thread-worm of the horse (_Ascaris_ _megalocephala_), which has become famous through the classical observations of Ed. van Beneden. Many favourable circumstances unite in this case to make the essentials of the process clearly recognizable. Fertilization takes place within the body of the female, in an enlarged portion of the oviduct, within which a number of the remarkable sperm-cells are always found in a mature female. They are remarkable in being not thread-like, but rather spheroidal cells, bearing, however, a small protuberance something like a pointed horn (Fig. 75, _A_, _sp_). When such a sperm-cell comes in contact with the upper surface of an ovum a swelling forms at the place touched, and the sperm-cell attaches itself firmly to this, and is drawn by it into the ovum. Without doubt, amœboid movements on the part of the sperm-cell itself play some part in this, as can be most plainly seen in the large sperm-cells of many Daphnids which we have already discussed. In the egg of the thread-worm the whole sperm-cell with its nucleus can soon be detected within the substance of the ovum, and it then changes rapidly. Its main body fades more and more completely, until at last it disappears altogether, while the nucleus becomes vesicle-like and soon attains a considerable size (Fig. 75, _B_, _spk_). Meanwhile the residue of the germinal vesicle which remained behind in the ovum after the second directive division (_B_, _Eik_) has changed into a large vesicle-like nucleus (_C_, ♀ _k_), which in the ovum of _Ascaris_, as well as in the spermatozoon, at first contains a nuclear reticulum with irregular fragments of chromatin. Later on, these form a spiral coil in the manner we have already described, and finally this breaks up into two large and relatively thick angular loops or chromosomes (Fig. 75, _C_ and _D_, _chr_).