374 OUTLINES OF CHORDATE DEVELOPMENT 



The presence of a vitelline membrane is not definitely known ; 

 surrounding the ovum, however, is a thick transparent mem- 

 brane apparently of chorionic nature (i.e., of follicular origin 

 secondary egg membrane), known as the zona pellucida. This 

 often has, either throughout or at least peripherally, the 

 appearance of being perforated by minute pores or canals, and 

 hence is often called the zona radiata. A micropyle is not 

 known. The zona pellucida is usually separated from the sur- 

 face of the fully grown egg by a narrow periritelline space. In 

 many Mammals, at the time the ovum escapes from the ovary 

 it is, and for a time remains surrounded by a few layers of 

 regularly arranged cells forming the corona radiata (Fig. 144). 

 This is a part of the ovarian egg follicle, and in order to under- 

 stand its relations we must outline the earlier ovarian history 

 of the ovum. 



The formation of the ova begins in the ovary before the time 

 of birth, and in the case of the Mammals all of the ova which 

 are to be produced during the period of fertility, are at that 

 time definitely established, although only partly differentiated. 

 In other words the period of the multiplication of the oogonia 

 is completed during embryonic life. Subsequently there occur 

 the phases of oogonial growth and the maturation processes. 

 In the embryonic ovary the primordial germ cells divide 

 repeatedly and soon form large numbers of cells arranged in 

 small groups or "nests" (Fig. 145). The cells composing these 

 have all had a similar history, but are destined to have very 

 different fates. In each group one cell enlarges and becomes 

 a definitive ovum, while its sister cells and their descendants are 

 to form the egg follicle. All of the stages in the history of 

 the formation of the ovum and follicle can be found in the ovary 

 of a fertile individual. 



The follicle appears first as a small group of flattened cells, 

 forming a single layer around the slightly enlarged central 

 oogonium (Fig. 145). As the follicle cells multiply they become 

 cubical and then columnar, forming a definite epithelium. As 

 the ovum increases steadily in size the follicle more than keeps 

 pare with it, so that as the follicular epithelium becomes three 



