MEIOTIC DIVISION 



71 



In the reducing, meiotic, or maturation division each daughter-cell 

 gets one or the otlier member of each pair of homologous chromosomes. 



In the case of the ovum the meiotic division usually occurs in the 

 formation of the first polar body, so that it and the reduced nucleus of 



the ovum have each " chromosomes. There is no further reduction in 



2 



the formation of the second polar body, which involves an ordinary 

 equation-division. The first polar body often divides into two. Thus 



Y^mT- -Vt/. 



la • • 





I 



\l 'V V V,, 



• • • 



IPi 



\f\ 



Fig. 35. — Oogenesis and Spermatogenesis. — After Boveri. 



I. and I. A. Primordial germ-cells. 



II. -IV. and II.A-IV.A. Multiplication of germ-cells (oogonia and sperma- 

 togonia). 



V. An immature full-grown egg-cell. 



VI. It gives off the first polar body (P.B.i) by a meiotic division, and the 



first polar body may divide again (i, 2). 



VII. The reduced oocyte gives off a second polar body (3) by an equation 



division, and thus becomes the ripe egg (4). 



V.A. A spermatogonium which divides by a meiotic division to form two 

 spermatocytes (VI. a). 



Each of these divides again by an equation division, forming four sperma- 

 tids, which differentiate into spermatozoa (1A-4A). 



the result is one viable cell (the mature ovum) and three non-viable 

 cells (the polar bodies), each with - chromosomes. 



In the spermatogenesis or production of spermatozoa the meiotic 

 division is usually the second-last. A " mother-sperm cell " or 

 spermatogonium divides into spermatocytes with n chromosomes, each 



of these divides into 2 spermatocytes with ^ chromosomes, and these 



