THE DETERMINATION OF SEX 447 



conversely, that more males were killed by a rich diet while the 

 females survived. The investigators were therefore misled by 

 not taking into account the sex of the indi\aduals that died. In 

 addition to such destructive criticism of the older theory, it has 

 been found that the sex of many animals appears to be determined 

 as early as the one-cell stage by the combination of chromosomes 

 that the individual happens to receive at the time of fertilization. 



When this chromosome theory was first promulgated about 

 1905, it was supposed that the sex determination was irrevocably 

 settled at the outset and could not be changed by anything that 

 might happen in the subsequent history of the individual. Later 

 investigation has shown that modifications may occur in some 

 exceptional cases, but the essential fact that sex is related to the 

 number of chromosomes now seems thoroughly estabhshed. The 

 account that follows will begin with an explanation of the funda- 

 mental relationship, and will conclude with a statement of the 

 modifications of the theory that have been necessitated by recent 

 investigations. 



It will be recaUed (c/. Fig. 113, p. 226, and Fig. 114, p. 228) 

 that in the diploid grouping the chromosomes occur in pairs and 

 that one member of each of these has descended from the chromo- 

 somes of the ovum and the other member from those of the sper- 

 matozoon. In maturation the members of these chromosomal 

 pairs are separated, so that each gamete receives one meml)er of 

 each pair and therefore one-half the number otherwise found in 

 body and germ cells, thus producing the haploid number. Hence, 

 before the "sex" chromosomes were recognized it was supposed 

 that the diploid number was always even, and the haploid number 

 necessarily one-half of the diploid number. The chromosome 

 theory of sex was first suggested when it was observed ftiat not all 

 the cells of a given species possess an even number of chromo- 

 somes in the diploid phase of the cell cj'cle. For example, in some 

 insects, it was found that the cells of the male, as seen in the 

 spermatogonia (Fig. 113), possessed one chromosome less than the 

 oogonia (Fig. 114). This odd chromosome was first called the 

 "X-chromosome," because its significance was unknown, and later 

 the sex chromosome when it came to be identified with the mechan- 

 ism of sex determination. In contrast with the sex chromosomes, 

 the other chromosomes are known as autosomes. Fig. 113 shows 

 how the unpaired sex chromosome of the male divides at every 



