18 



TEXT-BOOK OF EMBRYOLOGY. 



(Fig. 9, 1, J and K). The somatic number of chromosomes has been reduced 

 -half, and the egg nucleus— female pronucleus— -is ready for union with the 



one 



sperm nucleus — male pronucleus. 



Since the chromatin rods left after the formation of the polar bodies are 



J 



AT 



Fig. 9. — Maturation of the ovum of Ascaris megalocephala (bivalens). Boveri, Wilson. A, 

 The ovum with the spermatozoon just entering at *" ; the egg nucleus contains 2 tetrads (one not 

 clearly shown), the somatic number of chromosomes being 4. B, Tetrads in profile. C, Tetrads 

 on end. D, E, First spindle forming. F, Tetrads dividing. G, First polar body formed, containing 

 2 dyads; 2 dyads left in the ovum. H, I, Dyads rotating in preparation for next division. /, 

 Dyads dividing. K, Each dyad divided into 2 single chromosomes, thus completing the reduction. 



