22 TEXT-BOOK OF EMBRYOLOGY. 



of the male. The oogonia contain the diploid number of chromosomes and 

 divide by ordinary mitosis. After several generations they pass through a 

 period of growth and are then known as primary oocytes. During the growth 

 period there occurs a condensation of the chromatin, and synapsis of the chro- 

 mosomes probably takes place at this time. The nucleus then resumes its 

 reticular structure. Following this the spireme is formed, preparatory to divi- 

 sion, and segments into the haploid number of chromosomes. From this stage the 

 process varies somewhat in different animals. In Ascaris, whose diploid num- 

 ber of chromosomes is four, both maturation divisions occur after the sperm 

 has entered the egg and lies embedded there as the male pronucleus (Fig. 14). 

 An achromatic spindle forms near the surface of the ovum and the two tetrads 

 go to the equatorial plane (Fig. 14, E). Each tetrad separates into two dyads, 

 and one dyad from each tetrad passes into a small mass of cytoplasm which 

 becomes detached from the egg cell as the first polar body (Fig. 14, F, G, H). 



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 (J.pn.) pronuclei. Sobotta. 



A new spindle forms without the return of the nucleus to the resting stage, and 

 each dyad divides into two monads. The second polar body is now given off in 

 the same manner as the first. One monad from each dyad passes into a small 

 mass of cytoplasm and is separated from the egg cell (Fig. 14, H, I,'J, K). 

 The maturation process is now complete. The nucleus of the mature ovum 

 contains the haploid number of chromosomes and is ready for union with the 

 male pronucleus. 



The maturation of the mouse ovum, recently described by Mark and Long, 

 may be taken as an example of mammalian maturation. The diploid number 

 of chromosomes is twenty, but when the growth of the primary oocyte is com- 

 pleted and the cell prepares for division only ten chromosomes are present. 

 Each chromosome is V-shaped and shows the structure of a tetrad. While 

 still in the Graafian follicle the first polar body is given off and lies as a small 

 globule beneath the zona pellucida (Fig. 13, A). The egg cell and the first 

 polar body constitute secondary oocytes, comparable with the secondary sper- 



