I have given (_General Morphology_, chap, ii., p. 104) the name of strophogenesis to the complicated process of cell-reproduction, which we find in the ontogeny of most of the higher histona, both phanerogams and cœlomaria. In these there is not a real alternation of generations, as the multicellular tissue-forming organism develops directly from the impregnated ovum. But the process resembles metagenesis in so far as the ontogenetic construction consists itself in a repeated division of the cells. Many generations of cells proceed by cleavage from the one stem-cell (the impregnated ovum) before two of these cells become sexually differentiated, and form a generation of sexual cells. However, the essential difference consists in the fact that all these generations of cells--in the body of both the higher animals and the flowering plants--remain joined together as parts of a single bion (a unified physiological individual); but in the alternation of generations each group produced is made up of a number of bionta, which live as independent forms--often so different from each other that they were formerly thought to be animals of separate classes, such as the polyps and medusæ. Hence we must not describe the reproductive circle of the phanerogams as an alternation of generations, although it has started from the fern (by abbreviated heredity). All simple forms of sexual reproduction without alternation of generations are comprised under the title of _hypogenesis_. The generative cycle proceeds from ovum to ovum in one and the same bion or physiological individual. This form of development is usual with most of the higher animals and plants; it may proceed with or without metamorphosis. The younger forms which arise temporarily in the latter case, and are distinguished from the sexually ripe form by the possession of the provisional (and subsequently disappearing) organs--larva organs (for instance, the tadpole or the pupa), are comprised under the general head of larvæ.