THE DETERMINATION OF SEX 209 



may be either single or multiple. It may be paired 

 before maturation with its absence, or with an unlike 

 ("?/") chromosome. It may be linked inseparably 

 with some one of the ordinary chromosomes (auto- 

 somes), or resemble the autosomes so closely that its 

 presence can only be assumed from analogy with 

 other cases, and not definitely determined at all. 



In all of these cases, however, there is one point 

 of likeness, and that is that there always seems to 

 be additional chromatin material associated with the 

 female sex. 



The reason for this may lie in the more highly 

 metabolic requirements of the female, who must 

 produce yolk or provide in some way for the main- 

 tenance of the young in addition to furnishing half 

 of the germinal heritage. 



In the microscopical evidence on this point there 

 is one apparent exception to the rule that females 

 are homozygous and males heterozygous with respect 

 to sex. Baltzer (1910) found that in one of the sea- 

 urchins an extra sex chromosome is associated with 

 the female sex, so that two kinds of mature eggs are 

 produced upon maturation ' and only one kind of 

 sperm-cells. In other words, in this case the female 

 is heterozygous for sex and the male homozygous, 

 instead of the reverse which is true for all other 

 forms thus far microscopically investigated. 



Such cases as this of the sea-urchin are theoreti- 

 cally provided for in the formulae under type I given 

 above in Figures 59, 60 and 61. 



