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INTRODUCTION TO CYTOLOGY 



In the fruit fly, Drosophila melanogaster (Stevens 1907; Morgan 1911; 

 Metz 1914) (Fig. 140), there are four pairs of chromosomes, including in 

 the male an XY pair and in the female an XX pair. Reduction in 







FIG. 139. Sex-chromosomes in various insects. 



A, spermatocyte of Lygaeus, showing the .X-chromosome at left and y-chromosome 

 above, both split. X 2250. (After Wilson.) B, prophase in spermatocyte of Prionidus, 

 showing sex-chromosomes enclosed in plasmosome. X 2294. (After Payne.) C, pro- 

 phase in spermatocyte of Protenor. X 2250. (After Wilson.) D, metaphase of hetero- 

 typic mitosis in spermatocyte of Protenor. X 2250. (After Wilson.) E, anaphase of 

 heterotypic mitosis in spermatocyte of Musca domestica; h, heterochromosomes. X 1500. 

 (After Stevens.) F, the two daughter chromosome groups in the anaphase of the hetero- 

 typic mitosis in the oocyte of Phragmatobia fuliginosa, showing 28-29 distribution. X 4080. 

 (After Seiler.) 



spermatogenesis gives sperms of two sorts: all contain four chromosomes, 

 but in half of them one of the four is the X, and in the other half it is 

 the Y. Since every egg contains an X, two kinds of union are possible 

 at fertilization : an X with a Y, giving a male fly, and an X with an X, 



