84 COLLEGE ZOOLOGY 



Fig. 48; Fig. 49, a). At the completion of the growth period 

 they are termed primary oocytes (Fig. 49, b). The primary 

 oocytes contain only one half the number of chromosomes char- 

 acteristic of the somatic cells and oogonia. As in the primary 

 spermatocytes, these chromosomes are bivalent, resulting from 

 the union two by two of the univalent chromosomes of the 

 oogonia. The primary oocyte divides in the following manner. 

 Its nucleus, called the germinal vesicle (Fig. 49, a), moves to the 

 periphery (b), where a mitotic figure is formed perpendicular . 

 to the surface of the egg (c). A small bud-like protrusion is now 

 formed into which pass one univalent chromosome from each of 

 the bivalent chromosomes present in the primary oocyte (d). 

 The bud is then pinched off. Two secondary oocytes are pro- 

 duced by this division, each containing an equal amount of 

 chromatin, but one with a great deal more cytoplasm and yolk 

 than the other (e) . The small one is known as the first polar 

 body (e, p.b. 1) and is not functional; the larger is the egg. 

 Each secondary oocyte now prepares for division (e). The first 

 polar body in some cases does not divide; when it does, the divi- 

 sion is equal (g, p.b. 1). The egg throws off a second polar body 

 (g, p.b. 2), which contains one half of each chromosome. This 

 second polar body disintegrates, as does the first. 



(3) Fertilization. — The mature ovum now becomes the center 

 of the interesting process of fertilization. The spermatozoon 

 sometimes enters the egg before the polar bodies are formed, and 

 sometimes afterward. In the illustration (Fig. 49, e) the sperm 

 is shown entering the egg at the end of the first oocyte division. 

 The sperm brings into the egg a nucleus, a centrosome, and a 

 very small amount of cytoplasm. The sperm nucleus soon 

 grows larger by the absorption of material from the cytoplasm 

 of the egg, and the centrosome begins its activity. A mitotic 

 figure soon grows up (g) and moves toward the center of the egg. 

 The egg nucleus also moves in this direction (h) , and finally both 

 the male and female nuclei are brought together in the midst 

 of the spindle produced about the sperm nucleus (i). This 



