Although the poetic fancies we have mentioned as to the origin of the individual human soul are still widely accepted, their purely mythological character is now firmly established. The deeply interesting and remarkable research which has been made in the course of the last twenty-five years into the more minute processes of the impregnation and germination of the ovum has made it clear that these mysterious phenomena belong entirely to the province of cellular physiology (cf. p. 48). Both the female element, the ovum, and the male fertilizing body, the sperma or spermatozoa, are _simple cells_. These living cells possess a certain sum of physiological properties to which we give the title of the "cell-soul," just as we do in the permanently unicellular protist (see p. 48). Both germinal cells have the faculty of movement and sensation. The young ovum, or egg-cell, moves after the manner of an amoeba; the minute spermatozoa, of which there are millions in every drop of the seminal fluid, are ciliated cells, and swim about as freely in the sperm, by means of their lashes or _cilia_, as the ordinary ciliated infusoria (the flagellata). When the two cells meet as a result of copulation, or when they are brought into contact through artificial fertilization (in the fishes, for instance), they attract each other and become firmly attached. The main cause of this cellular attraction is a chemical sensitive action of the protoplasm, allied to smell or taste, which we call "erotic chemicotropism"; it may also be correctly (both in the chemical and the romantic sense) termed "cellular affinity" or "sexual cell-love." A number of the ciliated cells in the sperm swim rapidly towards the stationary egg-cell and seek to penetrate into it. As Hertwig showed in 1875, as a rule only one of the suitors is fortunate enough to reach the desired goal. As soon as this favored spermatozoon has pierced into the body of the ovum with its head (the nucleus of the cell), a thin mucous layer is detached from the ovum which prevents the further entrance of spermatozoa. The formation of this protective membrane was only prevented when Hertwig kept the ovum stiff with cold by lowering the temperature, or benumbed it with narcotics (chloroform, morphia, nicotine, etc.); then there was "super-impregnation" or "poly-spermy"--a number of sperm-threads pierced into the body of the unconscious ovum. This remarkable fact proved that there is a low degree of "cellular instinct" (or, at least, of specific, lively sensation) in the sexual cells just as effectively as do the important phenomena that immediately follow in their interior. Both nuclei--that of the ovum and of the spermatozoon--attract each other, approach, and, on contact, completely fuse together. Thus from the impregnated ovum arises the important new cell which we call the "stem-cell" (_cytula_), from the repeated segmentation of which the whole polycellular organism is evolved.