i8o THE POPULAR SCIENCE MONTHLY 



much speculation and but little work and to which experimental inves- 

 tigation 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 " Ausgangs- 

 zellen," from which an animal develops. Morgan 2 and Chambers 3 found 

 that small eggs of the frog give rise to smaller tadpoles and to smaller 

 frogs than do large eggs. Popoff* 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 con- 

 tinues 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 char- 

 acter, 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 



2 Morgan, ' ' Relation between Normal and Abnormal Development, etc., ' ' 

 Arch. Entw. Mech., 18, 1904. 



3 Chambers, ' ' Einfluss der Eigrosse, etc., ' ' Arch. mile. Anat., 72, 1908. 

 4 Popoff, " Experimented Zellenstudien, " Arch. Zellforschung, 1, 1908. 



