Relation Between Germ Cells and Envhonment. 251 



We may also point to the experiments in sex determination 

 among amphibians as carried out by Miss King and by 

 Richard Hertwig and his students. One difficulty with am- 

 ]>hibian material is the long delay in sexual differentiation in 

 certain strains. These very conditions point strongly toward 

 sexual indifference which manifests itself in the hermaph- 

 roditic tendency to be observed in the toad. Is it not possible 

 that the primitive germ cells, at least in these cases, may be 

 truly indifferent? The differentiation that appears later 

 might be brought about by the action of delicately balanced 

 internal and external factors. Color is lent to this view by the 

 fact that in many forms the secondar\^ sexual characters 

 develop in response to stimuli coming from the gonads, even 

 when germ cells are entirely lacking in the latter. Why should 

 we consider the differences between ova and sperm to belong 

 to a category apart from the "so-called" secondary' sexual 

 characters ? 



In the minds of many the problem of sex determination 

 is practically settled in favor of the sex-chromosomes theorj'. 

 There seems now little doubt that this applies quite widely, if 

 not universally, among the insects. While the presence of sex 

 chromosomes has been demonstrated in many other types of 

 animals, and the union of a sex chromosome with one of the 

 other chromosomes in Ascaj'is has suggested that it may thus 

 escape obser\'ation in still other forms where it has not been 

 demonstrated, the fact remains that there are many forms 

 where it has been most diligently sought without success. 



In organisms where the sex chromosome is clearly demon- 

 strable in each somatic cell, we should suppose that it could 

 influence its cell in each case toward its respective line of 

 sexual differentiation. For this reason we need not feel sur- 

 prised to find in the experiments of Kellogg, Harms, Messen- 

 heimer and Kopec upon lepidopterous larvae the clearest dem- 

 onstration that the secondary sex characters in those forms 

 are wholly independent of any influence from the gonads. This 

 is in sharp contrast to the experiments upon representatives of 

 most of the other types of animals, notably the vertebrates, 

 in which the gonads exert a profound influence upon the body 

 as a whole. 



It is not contended that the importance of the gonad as an 

 organ of internal secretion in a given species proves the absence 



