FUNDAMENTALS OF CYTOLOGY 



known to have s(>x chromosomes (Fig, 136). Hero the female plants 

 (gametophytes) ha\'e in their single genome one large X, while the male 

 plants (gametophytes) have a very small Y. After syngamy the zygote 

 with XY develops into the asexual sporophj^te. 

 When meiosis occurs at sporogenesis, disjunction of 

 the X and Y results in the presence of two spores 

 with X and two with Y in each quartet. These de- 

 velop, respectively, into new female and male game- 

 tophytes. It is of interest to compare this case with 

 that of the angiosperm in the preceding paragraph. 

 In the liverwort X and Y are correlated, respectively, 



J 9k h mth femaleness and maleness in different gameto- 

 I f phytes and gametes, with XY in the asexual phase 



9 • of the cycle; whereas, in the angiosperm the male 



gametophytes and gametes have either X or F, this 

 difference determining in turn which of the sporo- 

 phytes shall be female (XX) and which male (XT). 

 A characteristic feature of sex chromosomes is 

 their degree of heteropyknosis (page 86). About 

 half the X-chromosome in Drosophila is heterochro- 

 matic, the numerous genes that exert an influence 

 toward femaleness being arranged all along the euchromatic portion. In 

 some organisms practically the entire sex chromosome is heterochromatic 

 and is visililc during the metabolic stage as a dense, stainable body in 

 the midst of the chromonemata of the other chromosomes. Experiments 

 have shown that in Drosophila the Y exerts practically no influence upon 



Fig. 133.— A' 1- 

 chromosoine pairs 

 disjoining in first 

 meiotic divi.sion. a, 

 in spermatocyte of 

 man. b, XY pairs in 

 opossum, monkey, 

 and man. (After T. 

 S. Painter.) 





Fiu. 134. — ooiuatic chromosome comple- 

 ments from staminate (a) and pistillate (b) 

 individuals of a dioecious angiosperm {Melan- 

 drium album). (After M. Westergaard.) 







Fiu. 13."). Mctaphaso of first 

 division in microsporocyte of 

 drium. XY pair about to 

 (Photograph by H. E. Warmke.) 



meiotic 

 Melaiv- 

 disjoin. 



the kind of sex developed, although it does influence fertility. In 

 Melandrium, a genus of angiosperms, on the other hand, the large )' 

 seems clearly to exercise a strong influence toward maleness. Another 



