much speculation and but little work and to which experimental investigation might well be directed with promise of important results.
2. Another supposed factor which is not precisely hereditary nor yet strictly environmental is the size of the germ cells, of the "Ausgangszellen," from which an animal develops. Morgan[1] and Chambers[2] found that small eggs of the frog give rise to smaller tadpoles and to smaller frogs than do large eggs. Popoff[3] maintains that spermatozoa as well as ova vary in size, owing to slight inequalities of division during the genesis of these cells, and he supposes that when a large egg is fertilized by a large spermatozoon a large individual results, whereas if the sex cells are smaller than usual the individual developing from them will also be smaller. In favor of this hypothesis may be cited the fact that small eggs of Rotifera, Phylloxera and Dinophilus give rise to small and rudimentary males, whereas the larger eggs give rise to relatively large females. Within the same species, where the mode of development is the same for all individuals, egg size may be a factor in determining body size, but it is a relatively unimportant factor, since the size of an animal depends not merely upon its initial size, but chiefly upon the rate and duration of its growth. In many cases the smaller egg continues to grow for a longer period than does the larger one and in the end gives rise to a larger adult. This is strikingly shown in different species of Crepidula, where species with small eggs give rise to large animals and those with large eggs give rise to small animals. The large eggs produce large embryos, and the small eggs small embryos, but the latter continue to grow for a much longer period than the former and in the end give rise to animals of much larger body size than those which come from the large eggs. An egg of C. fornicata is about one quarter the volume of one of C. convexa, but the adult female of the former species is about 32 times the volume of one of the latter species, while the males of the former species are 125 times the volume of those of the latter species. Other cases of a similar sort are known and they show that in different species egg size can not be correlated with body size, and even within the same species it is a relatively unimportant factor in determining size.
3. It is well known that many extrinsic factors influence the cliaracter, rate and duration of metabolism, and consequently the size of organisms. Among these extrinsic factors I shall mention only a few which are known to be important, viz., (a) quantity and quality of food, (b) secretions of certain glands, particularly the sex glands, thymus, thyroid and hypophysis, (c) various chemical substances, such
