SEXUAL DIFFERENTIATION 



357 



female organs and gametes. The sporophyte, which itself has no 

 sex differentiation, is therefore the sex heterozygote. In species 

 of Sphcerocarpus there are a large X chromosome and a very small 

 Y chromosome which pair at meiosis in the diploid sporophytes by 

 a terminal chiasma, and pass to opposite poles. The four spores 

 of the tetrad formed stick together until germination. They 

 produce two female gametophytes with X chromosomes and two 

 male gametophytes with Y chromosomes. Sexual differentiation 



ri^/^ 



9 



Fig. 109. — First metaphase in the two sexes of Alacronenmrns 

 appendiculatiis. XX not recognisable in the female, which 

 shows lower spiralisation and terminalisation. A' and Y lying 

 unpaired on opposite sides of the plate in the male. X 1600. 

 (Naville and de Beaumont, 1933.) 



is therefore determined by the segregation of these X and Y chromo- 

 somes at meiosis (Allen, 1919, 1935 h). 



In the higher animals and plants it is in the first instance the 

 diploid generation which is sexually differentiated. We then find, 

 as a rule, that one sex is heterozygous, with an XY pair of chromo- 

 somes, while the other is homozygous, having a pair of similar 

 chromosomes XX. The convention, it will be noted, is necessarily 

 different. In the Bryophyta, X and Y are associated with the 

 female and male sexes respectively. They have exactly corre- 

 sponding life cycles. In the higher plants and animals they have 

 not. X meets an identical mate in a sex homozygote (which does 

 not occur in the Bryophyta). Y, on the other hand, never pairs 

 with any chromosome but X, and therefore resembles in its genetic 



