GAMETOGENESIS AND SPOROGENESIS 



213 



Oogenesis in Animals. — The development of the animal egg varies 

 considerably in detail in the different groups. The general features of the 

 process as it occurs in typical cases may be briefly sketched as follows. ^° 

 In the ovary certain cells known as oogonia enlarge somewhat and become 

 primary oocytes. When first distinguishable the oocyte is relatively 

 undifferentiated, but it soon passes through a growth period during which 

 it enlarges greatly and becomes supplied with one or more kinds of yolk 

 (Fig. 128). A role in the elaboration of these products has been assigned 

 to the chondriosomes and Golgi material by a number of investigators 

 (pp. 78 to 88). In some animals the changes during the growth period 



Fig. 128. — Differentiation of oocyte in Hydra. A, very young oocyte between ecto- 

 dermal cells at right. B, oocyte after growth period, with yolk globules. {After Downing, 

 1909.) 



involve the activity of a special follicular epithelium organized about the 

 oocyte, or a group of "nurse cells." The oocyte at this stage is often 

 called the "ovarian egg" and its large nucleus the "germinal vesicle." 



At the close of the growth period the oocyte nucleus, which is already 

 in an advanced stage of the prophase, undergoes division near the cell 

 membrane. One of the daughter nuclei so formed is included in the first 

 polocyte, or "polar body," a small cell cut off at this point (Fig. 154). 

 The nucleus remaining in the large cell, now called the secondary oocyte, 

 quickly undergoes another division, one of the resulting nuclei being 

 included in a second polocyte, or "polar body," while the other remains in 

 the now mature ovum, or egg. The first polocyte may divide and thus 

 complete a quartet of cells, but often this does not occur. Normally the 

 polocytes are functionless and soon degenerate, sometimes within the 



20 See Hegner (19146), Wilson (1925), and Corner (1932). The question of the 

 origin of the germ cells is reviewed by Harms (1926) and Heys (1931). 



