308 



GENERAL EMBRYOLOGY 



mosome, although univalent, divides just as the ten bivalent 

 elements do, and eleven chromosomes consequently pass into 

 the nuclei of the secondary spermatocytes. But during the 



FIG. 142. Maturation during the spermatogenesis of the squash-bug, Anasa 

 tristis, showing the behavior of the X-chromosome or idiochromosome. A, after 

 Wilson, others after Paulmier. A. Spermatogonium. Polar view of equatorial 

 plate showing twenty-one chromosomes (ten pairs, plus one). The X-chromo- 

 some is not distinguishable at this time. B. Primary spermatocyte. Tetrads 

 formed. C. Equatorial plate of first spermatocyte division. X-chromosome 

 divided. D. Anaphase of same division. The daughter X-chromosomes have 

 also diverged. E. Equatorial plate of second spermatocyte division. F. M eta- 

 phase of same division. The X-chromosome lies, undivided, between the two 

 groups of daughter chromosomes. G. Anaphase of same division. The undi- 

 vided X-chromosome has passed to the upper pole, lagging behind the others. 

 H. Telophase of same division. X-chromosome still distinct. 



mitosis of these secondary spermatocytes the idiochromosome 

 fails to divide and passes as a whole to one pole of the spindle 

 (Fig. 142, F, G, H). The result is that the nuclei of one-half 

 of the spermatids, and therefore of one-half of the spermatozoa, 



