The human spermatozoön is a living cell: it has (1) the requisite structure; (2) the chemical composition of an organic being; (3) a figure in keeping with its species; (4) an origin from a living progenitor; (5) the _explicatio naturae_; (6) the power of assimilation; (7) the _duratio viventium_; (8) the power of reproduction; (9) motion and locomotion. As soon as the ovum breaks through the surface of the ovary it has all the qualities of the spermatozoön except locomotion. These two cells are animal cells, not vegetable; just as single-celled protozoa, like Actinophrys, Actinosphaerium, Closterium, Stentor, and the Amoebas are animals, not plants. It is not possible in our present knowledge sharply to differentiate ultimate forms of plants from animals. To say that animals have the qualities of plants plus a sentient vital principle is not enough. It is very doubtful that even the so-called sensitive plants feel, and it is practically certain that many low forms of animal life do not feel--they have no sentient mechanism. Plants have the qualities enumerated above plus the power of drawing nutriment directly from inorganic material, while animals can draw nutriment directly only from organic material; yet some fungi, bacteria for example, will grow and thrive only on organic material, and animals will take up mineral drugs. It is questionable, however, that minerals which thus find a way into animal cells are really assimilated. They excite or irritate these cells into intenser action, and thus cause growth, rather than affect development by direction. The so-called mineral tonics used in medicine act by irritation. This irritation or stimulation by drugs can in certain very low forms of animal life start mitosis in the unfertilized ovum, and thus build up part, at the least, of a specific embryo parthenogenetically: here probably a polar body takes the place of the spermatozoön. Loeb, by treating the unfertilized egg of Arbacia (a sea-urchin) with magnesium chloride, started mitosis that resulted, it is said, in a perfect Pluteus larva.[34] [34] _American Journal of Physiology_, 1899, iii, 3. The human ovum is about half the size of a period in the type of this page, and two hundred and fifty spermatozoa will fit side by side along the horizontal diameter of the lowercase letter _o_ here. The nuclei of these cells are extremely minute: they must be stained and be observed with a high-power objective on the microscope before they become visible. This small nucleus of the spermatozoön penetrates the covering membrane of the ovum, enlarges, and becomes the male pronucleus. The pronucleus unites permanently with the pronucleus of the ovum, and together they form the Cleavage or Segmentation Nucleus of the fertilized ovum. This new nucleus gives rise by division to the innumerable myriads of nuclei in the growing body. Hence every nucleus of the child apparently contains nuclear material derived from both parents, as has been said.