MATURATION OF THE GERM-CELLS 169 



side of the aorta. Here they divide many times to form 

 oogonia. Their mode of division is the indirect one (karyo- 

 kinesis), and is accompanied by the formation of a mitotic figure 

 as described on pp. 165-6 for a tissue-cell. Each oogonium 

 contains the same number of chromosomes as does each tissue- 

 cell. The whole mass of such oogonia, together with the blood- 

 vessels, connective tissue, nerves, and the peritoneal covering 

 associated with them, constitute the ovary. 



These germ- cells, however, are not mature, nor are they 

 capable of giving rise to a new generation of individuals until 

 they have passed through (a) a period of growth, (6) a process 

 of maturation, and (c) a process of fertilisation. 



Shortly before the first batch of eggs is laid, a certain 

 number of oogonia begin to store up yolk in their cytoplasm. 

 This stage is the period of growth. In the frog about 3000 

 oogonia undergo this change during the winter. Such large 

 immature germ-cells may be called primary oocytes, whilst the 

 remaining oogonia are reserved for a subsequent spawning 

 period. 



The next stage is occupied by the process of maturation or 

 ripening. This process consists in the extrusion of nuclear and 

 cytoplasmic substances from the oocyte in the form of two 

 corpuscles known as the polar bodies. The nucleus of the 

 oocyte moves to the surface. A mitotic figure is developed 

 which differs, however, in an important aspect from the mitotic 

 figure of the tissue-cells and of dividing oogonia. The chromo- 

 somes are fused in pairs so that only half the full number is 

 present. In other respects the division of the spindle and 

 chromosomes takes place as described on p. 166. Each double 

 chromosome divides transversely, one half going to each pole 

 of the spindle. As the spindle with its attached chromosomes 

 divides, one half passes to the exterior of the oocyte together 

 with a small amount of cytoplasm and is pinched off as a small 

 cell the first polar body. The other half of the spindle now 

 retires into the interior of the oocyte, and its chromosomes are 

 reconstituted into a fresh nucleus. 



After an interval this nucleus repeats the process, and the 

 second polar body is extruded. This has again the reduced 

 number of chromosomes. In the case of certain animals the 

 first polar body meantime has divided into two. The remain- 



