SEX DETERMINATION 2O9 



genotype, the germ cells developed in the grafted male gonad into 

 sperm. Similarly, in Drosophila,^ a case has been found in which after 

 X-raying a female gave X and no-X eggs ; clearly one was lost from 

 some of the presumptive germ-cells, leaving them XO, genotypically 

 male, but in spite of this they developed as eggs since they were in an 

 ovary. 



Gamete sexuality in animals is therefore not a genetic question but 

 an embryological one. So is the further question (which cannot yet be 

 answered) of whether all cells of an embryo are potentially capable of 



Fig. 98. The Induction of Gamete Sexuality. — a is a figure of an embryo of 

 Rana sylvatica in the tailbud stage in lateral view; b is a transverse section. The 

 region within the dotted lines of 0, and between A and 6 in b, was exchanged 

 between different embryos. The germ cells at this stage lie in the dorsal ridge of 

 endoderm (black in b) and were not moved. In some cases the sex of the gonad 

 on the operated side was different to that on the normal side. In these the grafts 

 must have been made between embryos of different sexes. The germ cells always 

 developed according to the sex of the gonad in which they found themselves, 

 and therefore sometimes at variance with their genetic constitution. 



(From Witschi, after Humphrey.) 



being differentiated into gametes, or whether the competence for this 

 differentiation is from the beginning confined to a certain series of cells 

 constituting a definite germ-track. 



In organisms in which the sexuality is confined to the gamophase, 

 it is probable that the differentiation of the actual gametes is also often 

 an embryological rather than a genetical question (e.g. in the diploid 

 hermaphrodite moss gametophytes, p. 214). Only in those organisms 

 with an asexual zygophase and a sexual gamophase which consists 

 simply of the gametes themselves, does the gamete sexuality seem 

 to be determined directly by the factors contained in the gametes 

 (e.g. Fungi). 



Hartmann^ has shown that in some of the lower plants (e.g. in the 



^ Muller and Dippel 1926. Hartmann 193 1, 1932, 1934. 



