446 INTERNAL SECRETIONS 



after sexual differentiation of the soma has been already going 

 on, according to Lillie, for some time. 



Keller (1920) has examined the question of differences of sex 

 characters of male and female embryos in cattle at different 

 stages of about 25 to 67 cm. of body length. He finds that in 

 opposite sexed twins with both twins normal the male partner 

 is always longer and heavier than the female. All the measure- 

 ments relating to the skeleton and the muscles were greater 

 in the male than in the female. The sex differences already 

 begin to appear at the end of the second intrauterine month, 

 and become gradually more and more accentuated ; in the fifth 

 or sixth month they are no less developed than in mature 

 foetuses. Keller concludes that a neutral or asexual soma 

 could be held to exist in cattle only for a short period of early 

 embryonic development, but that such an assumption would 

 not imply that somatic sex differences are absent before visible 

 differences appear. 



A detailed criticism of our paper has been made by Kammerer 

 (19 19). He finds that our assumption is opposed by the fact 

 that the sex can be determined already at fertilization; we 

 shall discuss this question in the following section. Further, 

 he insists that our hypothesis is contrary to what we know 

 concerning the heterochromosomes ; this question is discussed 

 below. Kammerer holds that the soma is a bisexual one ; but 

 I do not see what difference there is between the assumption 

 that the soma, during embryonic development, is an asexual 

 one, and that the soma is a bisexual or an indifferent one, as 

 Kammerer holds. When speaking of an asexual soma I have 

 nothing else in mind than that the soma is an indifferent one 

 in the sense that either male or female sex characters can 

 develop according to the sex of the hormones produced by the 

 gonads. The assumption of Kammerer that our hypothesis 

 implies that sex characters are formed by sex specific hormones 

 "from absolutely undifferentiated plasm, and therefore, so to 

 speak, out of nothing" {Kammerer, 1919, p. 382), is incorrect. 

 According to our hypothesis sex characters are simply char- 

 acters of the neutral, asexual or equipotential form of a given 

 species, partly influenced and partly uninfluenced by sex 

 specific hormones. 



Kohn (1920) also has failed to realize that our h5^pothesis, as 

 already remarked, does not concern sexual determination, but 



