materials be added to the water inhabited by a dividing _Paramœcium_, the Protozoon may continue to reproduce by simple division for at least two thousand generations. We must remember, however, that “senescence” and “rejuvenescence” are only words; what is the essential nature of the changes denoted by them we do not know. In sexual reproduction, as it occurs in the great majority of plants and animals, the ovum, or female germ-cell, is fertilised or “activated” by the male germ-cell. But this activation by the spermatozoon is not necessary, for the ovum itself is capable of division and development to form a complete organism. This occurs in the cases of natural parthenogenesis among insects and some other animals, where the ovum proceeds, without fertilisation, to segmentation and development. In some lower plants, where the size of the male and female germ-cells is nearly equal, either of them may undergo parthenogenetic development: in such cases we cannot, of course, properly speak of sexual differentiation. In the cases of organisms normally reproducing sexually, the stimulus to development is afforded by the entrance into the ovum of the spermatozoon--that is, by the mixture of the male and female germ-plasms; but in some animals this stimulus may be replaced by the addition to the water in which they are living of certain chemical substances. This is the process of artificial parthenogenesis first studied by Loeb in the case of the eggs of the Sea-urchin; and its analysis suggests that the spermatozoon conveys some substance into the egg, and that this substance initiates segmentation by setting up a train of chemical reactions. What these reactions are exactly, and what is the process of “formative stimulation” by the spermatozoon, we do not know. It is quite certain, however, that much more than this process of formative stimulation is involved in the fertilisation of the egg by the spermatozoon. The mixture of the male and female germ-plasms resulting from conjugation confers upon the embryo the characters of both the parents and of their ancestries.