THEORY OF THE DEVELOPMENT OF ORGANISMS 123 



Males and females, whether they be more or less 

 unlike, arise from the same germinal material. 

 The germinal material itself is sexless ; that is to say, 

 there is not a male and a female germinal material. 

 The phenomena of inheritance in the sexual genera- 

 tion of hybrids show this clearly. Characters appro- 

 priate both to males and to females are transmitted 

 either by eggs or by spermatozoa. In partheno- 

 genetic animals both male and female individuals 

 appear at definite times from eggs produced without 

 sexual commerce. Whether the male or the female 

 forms be produced depends, not upon any difference 

 in the germinal material, but on the external influ- 

 ences, just as external influences determine whether 

 the bud on a twig shall give rise to a vegetative or 

 to a flowering shoot, to a thorn or to a stem. 

 The influence of food, of temperature, or probably 

 of other agencies, determines in which direction 

 the germinal material shall grow. 



The experiments of a distinguished French 

 investigator, M. Maupas, on the determination of 

 sex in Hydatina senta, a rotifer, have given 

 striking results. 



In Hydatina, under normal conditions the eggs 

 of certain individuals give rise always to males, of 

 others always to females. By raising or lowering 

 the temperature at the time when the eggs are being 

 formed in the germaria of the young females, the 

 experimenter is able to determine whether these 

 eggs shall give rise to males or to females. After 

 that early time the character of the egg cannot be 

 altered by food, light, or temperature. 



