382 INTRODUCTION TO CYTOLOGY 



fuse with each of them.^^ Binucleate eggs also appear to be responsible 

 for gynandromorphs in Hahrobracon.^'^ Gynandromorphism in the 

 katydid (Amblycorypha) may possibly be due to chromosomal elimina- 

 tion or to the entrance of two sperms, with X and Y, respectively, into 

 a binucleate egg. Polyspermy is known to occur in Drosophila, but its 

 relation to gynandromorphism has not been proved. ^^ 



Certain further facts are also in harmony with the view that the sex 

 of the individual is dependent upon the genetical constitution of the 

 protoplast with which development begins. In a number of animals 

 exhibiting parthenogenesis, sex is correlated with chromosome number 

 (p. 410). Human twins, if "identical" (produced by the same zygote), 

 are invariably of the same sex; if "fraternal" (produced by different 



A ^- ^ B 



Fig. 216. — A, a Drosophila gynandromorph, female on left side and male on right. 

 B, explanation suggested by Morgan for this condition: in an early cleavage division one 

 J^-chromosome fails to be included in either daughter nucleus, so that the nuclei in a 

 portion of the fly have XX while those in the remaining portion have X. (After Morgan 

 et al., 1922.) 



zygotes) they may or may not be of the same sex. In the nine-banded 

 armadillo one zygote commonly gives rise to four individuals, which 

 are invariably all male or all female. Similar examples of polyembryony 

 are known in insects. In the light of what has been set forth in the fore- 

 going paragraphs it appears probable that the constitutional factors 

 affecting sexual differentiation are largely in the chromosomes. 



Sex-chromosomes in Bryophytes. — In Sphoerocarpos, a genus of 

 heterothallic liverworts, two spores of a single quartet develop into male 

 gametophytes while the other two give rise to females. This is associated 

 with the behavior of a pair of sex-chromosomes, the first to be discovered 

 in plants'^ (Figs. 218, 219). The sporophyte has eight pairs of chromo- 

 somes, including a very unequal XY pair. At the time of sporogenesis the 

 X and Y disjoin in the first mitosis and divide in the second. The two 



IS Doncaster (1914), Goldschmidt and Fischer (1927), Goldschmidt and Katsuki 

 (1927). 



" Whiting (1931), Whiting and Stancati (1931). 



18 Pearson (1927, 1929) on Amblycorypha, Huettner (1924, 1927) on Drosophila. 



" C. E. Allen (1917, 1919). See also Schacke (1919), Wolfson (1927), and Lorbeer 

 (1930). The sexual tendencies of the four spores were discovered by Douin (1909) 

 and Strasburger (1909?>). 



