riddle: control of sex ratio 353 



and here, true to the rule that has been established elsewhere in 

 all this general line of work/ the resulting modification is cor- 

 respondingly strong and striking. When, by whatever means, 

 we effect a change in the metabolism (which is the essential 

 thing) at a still earlier stage — in the egg-stage in our own and 

 in some other experimental reversals of sex — then we obtain in- 

 dividuals whose sexual nature is quite thoroughly reversed; 13 in 

 many cases completely so, and in still other cases with varying 

 degrees of completeness. 



Baltzer's ('14) beautiful experiments with the worm Bonellia 

 best illustrate this fact and show the several stages of modifica- 

 tion not only in one and the same animal form but in the in- 

 dividuals hatching from a single brood. Baltzer finds that when 

 the larvae of this animal are hatched they are capable of becom- 

 ing either males or females. If they happen to become attached 

 to the proboscis of an adult female they become males; if they 

 do not succeed in so attaching themselves they soon settle from 

 the water into the sand or mud of the sea-bottom and there 

 undergo, quite slowly, further development into females (almost 

 exclusively). The plastic, reversible, quantitative nature of sex 

 in this form was shown by this investigator in the following 

 way: Some of the free-swimming "indifferent" larvae were 

 artificially helped to a connection with the proboscis of an adult 

 female. Some of these were permitted to maintain this attach- 

 ment for a very short period; others were removed at progres- 

 sively longer periods, with the very significant result that prac- 

 tically all stages of hermaphroditism were -produced. Those 

 first removed becoming almost perfect females, others with 

 longer and longer periods of attachment, becoming more and 

 more perfect males. 



Now the conditions under which the two sexes are here de- 

 veloped afford, in our own opinion, good reasons for believing 

 that the larva is stimulated — through its contact with the living 



13 The observations of Steche ('12) on the relation of precipitin reactions to 

 sex, as seen in the blood of insects are of much interest. This author thus finds 

 that male and female of the same species present differences as great as do the 

 males of two related species, or as do the females of related species. 



