72 



Special Chromosomes and Sex Inheritance 



Fig. 23. Meiosis in a male squash bug, Anasa tristis. The sex chromo- 

 somes in the female sex are XX and in the male XO. (a) Late prophase 

 of division of primary spermatocyte showing nine bivalents, one unpaired 

 X chromosome, and two small univalents which have not paired but will 

 pair as the spindle forms, (b) First meiotic metaphase in polar view; the 

 paired small chromosomes are in the center and the large X chromosome 

 is toward the outside of the principal ring, (c) First anaphase ; the small 

 chromosomes separate in advance of the others; the X chromosome 

 divides equationally at the first division and is shown separating to the 

 poles slightly later than the autosomes (which are dividing reductionally) . 

 (d) First telophase; the X chromosomes appear to be outside the prin- 

 cipal ring formed by the others, (e) Second metaphase in polar view. 

 (/) Second anaphase ; the X chromosome passes undivided to one pole but 

 later than the others; the equational splitting at the first division and the 

 passing reductionally at the second spermatocyte division have been termed 

 postheterokinesis. (g) Second telophase; one cell has nine large and one 

 small autosomes, and the other has nine large and one small autosomes 

 and one X chromosome. Xl^25. Camera lucida drawing. 



