This peculiar hermaphroditic formation of the cells, which distinguishes the ciliated infusoria and some other protists, and which we now know in its smallest details through the investigations of Richard Hertwig, Maupas, and others, is especially interesting because it proves that the chemical difference between the female gynoplasm and the male androplasm can be found within a single cell. This erotic division of labor is so important that formerly it was universally ascribed to two different cells. Recent accurate research, penetrating into the smallest visible processes of fertilization, has shown that the essential feature in the formation of a fresh individual (the stem-cell) is the blending of equal portions (hereditary parts) of the male and female nuclei; the caryoplasm of the two copulating cells is the vehicle of heredity from the parents. The cytoplasm of the cell-body, on the other hand, serves the purposes of adaptation and nutrition. As a rule the cell-body of the ovulum is very large, and is, as a food-store, very richly provided with albumin, fat, and other nutritive matter (food-yolk); while the cytoplasm of the sperm-cell is very small, and generally forms a vibrating lash, with which it moves along and seeks the ovum. In most of the plants the female and male cells are produced by the same sprout, and in many of the lower animals by one and the same person. This kind of hermaphrodism in "individuals of the second order" is called monoclinism ("one-beddedness"). In many of the higher plants (monœcic stocks) and most of the higher animals we have diclinism ("two-beddedness")--in other words, the one sprout or person has only male, and the other sprout or person only female, organs--this is gonochorism of individuals of the second order. Monoclinism is generally associated with sedentary life (and often necessary for it), and diclinism with free movement. Adaptation to parasitic habits also favors monoclinism; thus, the crabs, for instance, are for the most part gonochoristic individuals, but the creeping crabs (_cirripedia_), which have adopted sedentary (and to an extent parasitic) habits, have become hermaphrodites in consequence. Many intestinal parasites among the lower animals (such as tape-worms, suctorial worms, wonder-snails), which live isolated lives inside other animals, have to be hermaphroditic and able to fertilise themselves if the species is to be maintained. On the other hand, many hermaphroditic flowers, although they have both sorts of sex-organs, are incapable of fertilizing themselves and have to receive this from insect visitors which carry the pollen from one flower to another.