The Role of the Chromosomes 233 



and later become females. In the sand hopper (Orchestia) 

 occur alternate periods of maleness and (partial) femaleness, 

 while in Bonellia, one of the gephyrean worms, the larva 

 develops' into a male or female, depending upon whether it 

 lives attached or free. While sex appears, in many cases at 

 least, to be predetermined in the egg, it would not be safe 

 at present to say that this predetermination was in all cases 

 irrevocable. 



A curious condition has been discovered by Banta in some 

 of the daphnids (Daphnia and Simocephalus) in the form of 

 * * sex-intergrades. " In these animals the sexes are readily 

 distinguishable externally by several secondary sexual char- 

 acters, such as size and form of body and appendages, hair- 

 iness of ventral surface, etc. Banta has found a complete 

 series of intergrades, ranging from normal males on the one 

 hand, through males with one or more of the secondary char- 

 acters of the female, to hermaphrodites and females with 

 certain secondary male characters, and finally to normal 

 females on the other. A similar condition has been found 

 by Goldschmidt in the gypsy moth, and by Stout in the 

 common plantain weed. How far these results agree with 

 the chromosome hypothesis of sex determination is very much 

 "up in the air" at present. 



Reference has already been made to Hydra, a name classic 

 in biology as in mythology. Hydra may reproduce either 

 by simple division of its body into two parts, by forming 

 buds which develop into new Hydras and are then pinched 

 off the parent stock, or by developing sex organs followed by 

 fertilization. Whitney has shown that the appearance of 

 sex organs can be controlled by external factors, such as tem- 

 perature and food. Thus, if Hydra be kept for a time at a 

 low temperature, and the temperature be then raised and the 

 animal starved, testes and ovaries develop. Sexual repro- 

 duction in lower plants (i. e., Vaucheria, etc.) has also been 

 controlled by external factors. These latter experiments how- 

 ever deal with the control of sexual, as distinct from asex- 

 ual reproduction, and not with the determination of sex 

 itself. 



In the foregoing pages we have sketched very briefly a few 

 of the facts bearing upon the relative roles played by hered- 

 ity and environment in the development of the organism. 

 Needless to say the two factors are in no sense antagonistic 

 or mutually exclusive, but both work together in fashioning 

 the final product. The inheritance of the organism may be 

 compared to the molten metal; the environment, the mold 

 in which the metal is cast. 



