226 HEREDITY AND SEX 



recorded from the mounted museum specimen which 

 has the male plumage. But it is known that the re- 

 productive organs of hybrids, extreme as these, are gen- 

 erally imperfect and the birds are sterile. It has been 



Fig. 4. 







Fig. «. 







^■^^ 



Pig. 7. 



Fig. li 

 Fig. 8. FiR.a Fig- 10. Fig. 11. ^^., 



.^:r^-a^> ;>, '*^-: H»1 1^ i_rf 



p.:;.^^-i^::v, -^ m H ^ 



Fig. 108. — Oogenesis and spermatogenesis of Rhabditis aberrans. 

 1-5, stages in oogenesis, including incomplete attempt to form one polar 

 body. Eighteen chromosomes in 1 and again in 4 and 5. In 3 the entering 

 sperm seen at right. 6, prophase of first spermatocyte with 8 double and 

 two single chromosomes (sex chromosomes). At the first division (7) the 

 double chromosomes separate, and the two sex chromosomes divide, giving 

 ten chromosomes to each daughter cell (8). At the next division the two 

 sex chromosomes move to opposite poles, giving two female-producing 

 sperm (9 and 10). Rarely one of them may be left at the division plane 

 and lost, so that a male-producing sperm results that accounts for the rare 

 occurrence of males. (After E. Krixger.) 



shown that if the ovary of the female bird is removed 

 or deficient, she assumes the plumage of the male. 

 Possibly, therefore, some of these cases may fall under 

 this heading, but it is improbable that they can all be 

 explained in this way. In the cases examined by Guyer 

 himself the hybrids were dissected and all four were 

 found to be males. 



