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THE AMERICAN NATURALIST [Vol. XLIX 



has been demonstrated in certain cases that the egg con- 

 tains special organ-forming substances definitely located 

 in the cytoplasm, and that if these are removed definite 

 parts of the organism into which the egg develops will 

 be missing. We know, also, that the nucleus of the germ- 

 cell of either sex contains — at any rate, at certain 

 periods — a number of perfectly well-defined bodies, the 

 chromosomes, and these also have been definitely cor- 

 related in certain cases with special features of the adult 

 organization. 



^Before we can hope to complete our mental picture of 

 the manner in which organic evolution has taken place, 

 if only in outline, it is evident that we must be able to 

 account for the great complexity of structure which the 

 germ-cells themselves have managed to acquire, and also 

 to form some idea of the effect of this complication upon 

 the development of both the individual and the race. 



We must consider the origin of cytoplasmic and nuclear 

 complications of the egg separately, for they appear to 

 be due fundamentally to two totally distinct sets of fac- 

 tors. In the first place we have to remember that during 

 oogenesis the egg-cell grows to a relatively large size 

 by absorbing nutrient material from the body in which 

 it is enclosed. It is this nutrient material that is used 

 for building up the deutoplasm or food-yolk. There is 

 good reason for believing that the character of this 

 nutrient material will change, during the course of evo- 

 lution, pari passu with the changing character of the or- 

 ganism by which it is supplied. Doubtless the change is of a 

 chemical nature, for we know from precipitin experiments 

 that the body fluids of closely allied species, or even of 

 the two sexes of the same species, do exhibit distinctly 

 recognizable differences in chemical composition. It 

 also appears highly probable, if not certain, from such 

 experiments as those of Agar upon Simocephalus, that 

 substances taken in with the food, which bring about 

 conspicuous modifications of bodily structure, may at 

 the same time be absorbed and stored up by the egg-cells 



