CHROMOSOMES AND SEX 381 



The XO type is found in the chicken, the duck, and the pigeon.^- In 

 the chicken the number of small autosomes is uncertain, but the largest 

 chromosome of the complement is represented once in the female and 

 twice in the male (Fig. 215). In the pigeon the male has 62 somatic 

 chromosomes, including two large X-chromosomes; every spermatid 

 retains one X. Some embryos have 62, including XX, while others 

 have 61, including X. The latter are in all probability females. 



^?'<ct ^^v-c iiSUr:ir4 ^i^ 



01 a 



A B C 



Fig. 215. — Chromosomes in birds. A, diploid complement in male chicken; two 

 X-chromosomes (marked A). B, diploid complement in female; one X-chromosome. 

 {After Hance, 1924.) C, diploid complement in male pigeon, showing a XX pair (marked 

 aa). D, from a presumably female embryo, showing one X (a). (After Oguma, 1927.) 



In a genus of fishes, Platypoecilus, the genetic data indicate the presence 

 of an XF pair in the female. ^^ The fact that the nearly related genus 

 Lebistes appears on similar evidence to have the ZF in the male suggests 

 that one condition may be derived from the other, possibly by crossing- 

 over or translocation. This is further suggested by the occasional 

 appearance of XX males in Lebistes (Winge, 19306). 



Gynandromorphs afford further evidence of the role of chromosomes 

 in sex determination. Such individuals, with male characters in certain 

 parts of the body and female characters in other parts, are known in 

 several groups of animals which are normally unisexual, but most of them 

 occur among insects. Such a condition may arise in several ways. In 

 Drosophila the genetic evidence, including that in X-rayed flies, shows 

 that it is mainly due to the elimination of one X-chromosome during an 

 early mitosis in a female embryo (Fig. 216). Thus one part of the 

 body develops with X and the other portion with XX, the autosomes in 

 both portions being the same.^^ In the bee it is probable that gynan- 

 dromorphism results from the entrance of a sperm into one blastomere of 

 a divided egg.^^ In the Lepidoptera the morphological and genetical 

 data favor the theory that an egg may sometimes be formed with two 

 nuclei, with X and F respectively, and that a sperm nucleus may then 



12 Guyer (1909, 1916), Stevens (Boring, 1923), Hance (1924, 1926), and Akkeringa 

 (1927) on the chicken; Werner (1927) on the duck; Oguma (1927) on the pigeon. In 

 the turkey, Shiwago (1929) reports the XY type, the Y being very small. 



" Bellamy (1922, 1924), Gordon (1926, 1927). 



"T. H. Morgan (1914), L. V. Morgan (1929), Patterson (1931a). 



"Boveri (1888), Goldschmidt (1923a). 



