Moreover, we now know that these spermatozoa are not at all a peculiar form of cell; precisely similar cells are found in various other parts of the body. If they have many short threads projecting, they are called _ciliated_; if only one long, whip-shaped process (or, more rarely, two or four), _caudate_ (tailed) cells. Very careful recent examination of the spermia, under a very high microscopic power (Fig. 22 a, b), has detected some further details in the finer structure of the ciliated cell, and these are common to man and the anthropoid ape. The head (_k_) encloses the elliptic nucleus in a thin envelope of cytoplasm; it is a little flattened on one side, and thus looks rather pear-shaped from the front (_b_). In the central piece (_m_) we can distinguish a short neck and a longer connective piece (with central body). The tail consists of a long main section (_h_) and a short, very fine tail (_e_). Fig.22 A single human spermatozoon. Fig. 22—A single human spermatozoon magnified; a shows it from the broader and b from the narrower side. _k_ head (with nucleus), _m_ middle-stem, _h_ long-stem, and _e_ tail. (From _ Retzius._) The process of fertilisation by sexual conception consists, therefore, essentially in the coalescence and fusing together of two different cells. The lively spermatozoon travels towards the ovum by its serpentine movements, and bores its way into the female cell (Fig. 23). The nuclei of both sexual cells, attracted by a certain “affinity,” approach each other and melt into one. The fertilised cell is quite another thing from the unfertilised cell. For if we must regard the spermia as real cells no less than the ova, and the process of conception as a coalescence of the two, we must consider the resultant cell as a quite new and independent organism. It bears in the cell and nuclear matter of the penetrating spermatozoon a part of the father’s body, and in the protoplasm and caryoplasm of the ovum a part of the mother’s body. This is clear from the fact that the child inherits many features from both parents. It inherits from the father by means of the spermatozoon, and from the mother by means of the ovum. The actual blending of the two cells produces a third cell, which is the germ of the child, or the new organism conceived. One may also say of this sexual coalescence that the _ stem-cell is a simple hermaphrodite_; it unites both sexual substances in itself. Fig.23 The fertilisation of the ovum by the spermatozoon. Fig. 23—The fertilisation of the ovum by the spermatozoon (of a mammal). One of the many thread-like, lively spermidia pierces through a fine pore-canal into the nuclear yelk. The nucleus of the ovum is invisible.