366 PERMANENT HYBRIDS 



the Mammalia, on no more than a gain or a loss. Secondly, the 

 X and y chromosomes may regularly have complete terminahsation, 

 although the autosomes of comparable size retain interstitial 

 chiasmata at metaphase. Interstitial chiasmata have been found 

 in Pellia Neesiana (Tatuno, 1936) and in Humulus (Fig. 113). 

 They are rare in Rumex where the process of terminahsation has 

 been described (Sato and Sinoto, 1935)- Its completeness shows 

 that the pairing segments have been terminal and short, a part of 

 the chromosomes being differential. Thirdly, the X and Y chromo- 

 somes may regularly fail to form a chiasma in one arm, which must 

 then be differential, for when two X's are present in the female or 

 exceptionally two Y's in the male, chiasmata are formed in both 

 arms (Yamamoto, 1934, Fig. 114). If chiasmata fail in both arms 

 the whole chromosome is differential and the pairing segments 

 may be said to have disappeared. This extreme development is 

 found in many Hemiptera, e.g., Lygceus, Oncopeltus, etc. (Wilson, 

 1909, 1912), where the X and Y chromosomes may or may not 

 meet at pachytene. In others (Brochymene, Wilson, 1905 a) they 

 regularly meet. If they meet they fall apart again at diplotene 

 (i.e., they form no chiasma), divide equationaUy at the first division, 

 and segregate after momentary touching of their daughter halves 

 end-to-end at the second division. This extreme of differentiation 

 is found in Oncopeltus, where no size difference is observed between 

 the chromosomes, thus showing that mere loss and gain is only one 

 of the kinds of structural change that may arise to distinguish the 

 sex chromosomes. 



Table 56 



Representative Examples of Sex Chromosomes 



Plants. 



HepaticcB (undifferentiated sporophyte). 



XY Sphcsrocarpos Donnellii Allen, 1919 ; Lorbeer, 1927, 



1930. 

 XY S. texanus . . . Tinney, i935- 



XY Pogonatum inflexiim . Shimotomai and Koyama, 1932 



XY Pallavicinia longispina Tatuno, 1933- 



XY^Y^ Frullania spp. . . Lorbeer, 1934 {cf. Tatuno, 



1936). 

 XY 17 other species . . Lorbeer, I934- 



