106 AN INTRODUCTION TO ZOOLOGV 



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. 51, a), 

 moves to the periphery (b) where a mitotic figure is formed per- 

 pendicular to the surface of the egg (c). A small bud-like protru- 

 sion 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 produced 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. i) and is not functional; the larger is the egg. Each 

 secondary oocyte now prepares for division (/). The first polar 

 body in some cases does not divide; when it does the division is 

 equal (g, p. b. i). 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. 



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 illustrations (Fig. $i,/) we have 

 shown the sperm entering the egg at the end of the first oocyte 

 division. The sperm brings into the egg a nucleus, a centro- 

 some, and a very small amount of cytoplasm. The sperm nucleus 

 soon grows larger by the absorption of material from the cyto- 

 plasm of the egg, and the centrosome begins its activity. A mi- 

 totic 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 com- 

 pletes the process usually known as fertilization. In this process 

 the chief aim so far seems to be the union of two nuclei, one of 

 maternal origin, the other of paternal origin. We shall see later 

 that fertilization is really not consummated until the animal 

 which develops from the egg has become sexually mature. 



