48 



BIOLOGIC BASIS OF SEX 



terns although seldom in the structural ele- 

 ments of the body. This may be interpreted 

 in a variety of ways, including mutation 

 and various types of chromosomal inter- 

 change in either somatic cells or those in- 

 corporated into the germ cell tract. The 

 permanence of feather type differentiation 

 in grafts has been demonstrated by Dan- 

 forth (1932) and others. Conditions for sex 

 variability have certainly been demon- 

 strated as present in the bird (Hollander, 

 1944). Crosses between species of birds 

 have led to sterile hybrids and to rather 

 extensive discussions of sex reversal in the 

 development of such forms. In their hybrids 

 betweeen pigeons and ring doves. Cole and 

 Hollander (1950j presented a summary of 

 their evidence as well as a review of this 

 literature. Female hybrids rarely resulted 

 from pigeon sires mated to dove females 

 but were readily produced in the reciprocal 

 crosses. Surviving female hybrids produced 

 no eggs. Male hybrids produced abundant 

 sperm but varied greatly in the proportion 

 of sperm which seemed normal. In the back- 

 crosses to pigeons practically no fertility 

 was shown, but in back-crosses to doves 

 over 2 per cent gave fertile eggs and 9 of 

 these eggs hatched. All back-cross specimens 

 were males and have proven sterile, but tes- 

 tes and semen were not examined. With mi- 

 nor exceptions the expression of some 20 mu- 

 tant genes studied was not different from 

 that of the pure species. Recessives were not 

 expressed in the hybrids whereas most of 

 the dominants gave their customary pheno- 

 types. Sex-linked mutants were transmitted 

 as expected for each sex. This was particu- 

 larly important as the result of the inherit- 

 ance of these sex-linked characteristics 

 showed that sex reversal did not occur. 



Eggs from virgin hens are known occa- 

 sionally to undergo some development of the 

 germinal discs. In dark Cornish chickens 

 Poole and Olsen (1958) observed that some 

 parthenogenetic development was present 

 in 57 per cent of the eggs from their total 

 flock. Factors stimulatory to parthenogene- 

 sis were noticeably higher in hens of some 

 strains than in those of others. Birds within 

 the flock differed in jmrthenogenetic rates 

 from 100 per cent to 43 per cent. After incu- 

 bation for a 9- to 10-day period, 3.9 per cent 

 of the A strain, 0.7 per cent of the B strain, 



0.4 per cent of the C strain showed some 

 parthenogenetic development. In the ex- 

 perience of these observers these birds 

 showed more development than is found 

 generally in the domestic fowl. Yao and Ol- 

 sen (1955) showed that, in 95 per cent of 

 the instances when parthenogenetic de- 

 velopment was encountered, the growth con- 

 sisted solely of extra embryonic membranes. 

 In the remaining 5 per cent growth was 

 more advanced, ranging from the presence 

 of blood and blood vessels only, to the pres- 

 ence of well formed embryos in others. The 

 cells were diploid and were able to repro- 

 duce themselves by mitotic division. When 

 work on the turkey was begun in 1952, 17 

 per cent of the eggs began a limited cleav- 

 age on being incubated. Two embryos at- 

 tained the size of normal 3-day embryos. 

 In 1958, among 7269 eggs, 15 per cent were 

 found to contain blood of embryonic origin. 

 Embryos of various ages were encountered 

 in 9 per cent. Fifteen poults of partheno- 

 genetic origin were hatched in the 1958 sea- 

 son. No multinucleated or polyploid cells 

 were found in these turkeys. These poults, 

 as with others, have always been males. 

 Survival of parthenogenetic types generally 

 ranges from a few hours to several days. 



In ex]:>eriments with fowl pox it has been 

 shown that the number of eggs developing 

 parthenogenetically increases considerably 

 following vaccination. The factors leading 

 to parthenogenesis are considered to be the 

 genetic characteristics of the strain of birds 

 and the presence of an activating agent or 

 agents in the blood stream of the hens. 



The parthenogenetic forms are of particu- 

 lar interest to the problem of sex determina- 

 tion. The females should l)e producing two 

 types of oocytes Z -I- A and W -h A of 

 which presumably the Z + A alone survive 

 since the embryos capable of being sexed 

 are all males. The embryos are also diploids. 

 The 2Z -I- 2A could be derived from a fusion 

 of the Z -H A polav body nuclei as noted 

 earlier or possibly chromosome doubling 

 coming later in the early cleavage. A ge- 

 netic element seems partially to control the 

 parthenogenetic process. Chromosome dou- 

 bling would lead to cells with identical pairs 

 of chromosomes. The gene would be homo- 

 zygous. Inbreeding of poultry leads to a con- 

 tinning and rai^id loss in the vialiility of 



