FEMALE REPRODUCTIVE CELLS 



473 



to pole, the fibers do, according to Lams and Doorme, and Kirkham, 

 converge to points at which lie centrosome-like granules. 



The first maturation spindle bears 

 about 12 reduction chromosomes. The 

 number of chromosomes is yet a matter 

 of dispute. Lams and Doorme have 

 counted in two cases 12, and state the 

 number as varying from 12 to 15. Like- 

 wise Kirkham is inclined to make his 

 count 12. Sobotta gave the number as 

 12. 



The spindle at this stage usually lies 

 parallel to the surface of the ovum (Fig. 

 443); after metaphase the chromosomes 

 lie at the two ends of the spindle, which 

 has rotated so as to be at right angles to 



, r ,-,. i ,. FIG. 445. Ovum of mouse, show- 



the surface. Figure 444 shows this stage ing first ^^ bodV) sec0 nd polar 



spindle, and entering spermato- 

 goon. (After LAMS and DOORME.) 



in the maturation of the muskrat ovum. 

 The spindle fibers at this stage both in 

 the muskrat and mouse tend to be parallel straight filaments. 



The first polar body is formed as a bud from the surface of ovum 

 into which the distal end of the first maturation spindle travels. In this 

 position the spindle is divided, at the cell plate, by the constricting of 

 the bud to form the first polar body. 



The remains of the first maturation spindle, left within the ovum, 

 re-forms as a pointed spindle, which according to Lams and Doorme 

 becomes about the size of the first spindle. At its equator twelve or 

 more chromatin bodies assemble. As the second maturation spindle 



retreats, it also assumes a tangential 

 position (Fig. 445). 



The metaphase and anaphase 

 stages of the second reduction mitosis 

 ensue during the time that this sec- 

 ond spindle rotates at right angles to 

 the surface of the ovum. In a like 

 manner a second cytoplasmic bud is 

 formed, into which the distal half of 

 the second maturation spindle passes 



FIG. 446. Mature ovum of mouse, show- . t a- '^.-L. i_ 



ing first and second polar bodies, female to be Constricted Off With the SCpara- 

 pronucleus (/./.), and male pronucleus tion of the SCCOnd polar body. 



While the second maturation mito- 



sis is taking place, an entire sperm, according to Lams and Doorme, 

 and Kirkham, enters the cytoplasm of the ovum. 



