364 INTRODUCTION TO CYTOLOGY 



rate. Half the spermatids, and therefore sperms, receive nine chromo- 

 somes (8 + 1 accessory), while the other spermatids failing to receive an 

 accessory apparently degenerate. In the female there are 17 chromo- 

 somes: 16 autosomes and one accessory. Since the accessory passes 

 undivided to one pole at the first maturation mitosis and divides at the 

 second, half of the eggs receive nine chromosomes (8 + 1 accessory) and 

 half receive eight. An egg with nine fertilized by a sperm with nine 

 develops into a male with 18 (16 + 2 accessories). An egg with eight 

 fertilized by a sperm with 9 develops into a female with 17 (16 + 1 

 accessory). 



Cases of Parthenogenesis. — The cytological phenomena in those animals 

 reproducing in part by parthenogenesis (see p. 357) are of much interest 

 in this connection. In the honey bee the male, which develops from 

 an unfertilized egg, has the haploid number of chromosomes in his cells, 

 whereas the female, arising from a fertilized egg, is diploid. Similar in 

 some respects is the case of the gall-fly (Neuroterus), in which eggs that 

 have undergone reduction develop into haploid males, while other eggs 

 are formed without reduction and develop into diploid females. In the 

 male-producing eggs of Phylloxera there are two sex-chromosomes, two 

 others being lost in the polar body; in the female-producing egg all four 

 are present. Two kinds of sperms are produced, half of them with a 

 sex-chromosome and half of them without it. The latter kind degen- 

 erate, leaving only the former functional. All eggs, if fertilized, develop 

 into females. In Hydatina senta those eggs producing one polar body 

 and developing into females are diploid, whereas those giving off two 

 polar bodies and developing into males are haploid. In all of these 

 cases maleness accompanies the haploid, and femaleness the diploid 

 condition 1 . 



Plants. — Up to the present time a visible chromosome difference 

 between the two sexes in plants has been established only in Sphcero- 

 carpos, the genus of dioecious liverworts in which Douin (1909) and Stras- 

 burger (1909) found two of the spores of a single tetrad to be male and the 

 other two female. In Sphcerocarpos Donnellii (Allen 1917, 1919) (Figs. 

 142, 143) there are in the cells of the female gametophyte seven autosomes 

 which differ somewhat in length, and one very large X-chromosome. In 

 the cells of the male gametophyte there are seven autosomes and a very 

 small F-chromosome. The sporophyte therefore has eight pairs: seven 

 autosome pairs and the XY pair. Although all the stages in the divisions 

 at sporogenesis have not been seen, the evidence is sufficient to show that 

 the X and Y separate in the heterotypic mitosis and divide longitudinally 

 in the homceotypic. Two spores of a tetrad therefore receive an X- 

 chromosome in addition to the seven autosomes; these spores develop 

 into female gametophytes. The other two spores of the tetrad receive 



1 Compare the case of the frogs developing by artificial parthenogenesis, page 319. 



