n6 CYTOLOGY CHAP. 



intact to one pole, so that in anaphase I. there are three chromosomes 



/ 



B 



I 



\ 



H 









Jft 



N 







FIG. 53. 



The chromosomes in the life-cycle of Aphis saliceti. (After von Baehr, L.C., 1912.) A, spermatogonial 

 prophase ; B, primary spermatocyte, beginning of the meiotic phase ; C, D, E, F, G, evolution of the 

 definitive chromosomes. Note the two bivalents and the single X chromosome. H, metaphase I. ; I, ana- 

 phase I. ; J, K, telophase I. All the chromosomes now alike, all being univalent and split in preparation for 

 the second division. L, resting stage between the two divisions ; M, prophase II. ; N, metaphase II. ; O, 

 telophase II. (N and O illustrate the case of the spermatocyte II., which contains the X chromosome.) P, cell 

 from a segmenting egg with five chromosomes (i.e. a ) ; Q, cell from an embryo with six chromosomes (i.e. 

 a 9). 



p, plasmosome ; X, the sex chromosome. 



(2 + X) at one pole and only two chromosomes at the other, in the 

 familiar manner. When cell division takes place, however, this is unequal, 



