20 



TEXT-BOOK OF EMBRYOLOGY. 



primary chromatin rods. Tetrad formation, however, is of comparatively rare 

 occurrence, for in most animals, especially in the higher forms, no distinct 

 tetrads are formed. It is possible, however, that certain chromatin structures 

 in the primary oocyte may be analogous to tetrads. In the mouse, for example, 

 the most thoroughly investigated mammalian form, the spireme thread prob- 



m.pn. 



A 



FIG. 12. From sections of ova of the mouse, showing three stages in the maturation process. 



A, Ovum showing prophase of maturation division. /, fat; z.p., zona pellucida. 



B, Ovum showing maturation spindle with chromatin segments undivided. 



C, Ovum showing diaster stage of maturation division, formation of ist polar body (p.b.), and sperm 



nucleus (male pronucleus, m.pn.) just after its entrance. Sobotta. 



ably segments into one-half the usual number of pieces, but the further behavior 

 of these pieces differs from their behavior in Ascaris. Each chromatin seg- 

 ment is possibly equivalent to a tetrad; and the number of chromatin segments 

 is one-half the somatic number of chromosomes, just as the number of tetrads 

 is always one-half the somatic number of chromosomes. When the first 



f.pn. - 



m.pn. 



FIG. 13. From sections of ova of the mouse, showing the polar bodies (p.b.) and three stages of the 

 male (m.pn.) and female (f.pn.) pronuclei. Sobotta. 



maturation spindle forms in the ovum primary oocyte of the mouse, twelve 

 chromatin segments are found grouped in the equatorial plane (the somatic 

 number being twenty-four) (Fig. 10). Each chromatin segment next divides 

 transversely into two equal parts (Fig. n). One part from each segment now 

 passes out into a small globule of cytoplasm which becomes detached from the 



