GAMETES AND. FERTILIZATION II 



otherwise it is likely to be retained until the following day. 

 This over night retention of the egg is one of the factors which 

 accounts for the variability in the stage of development reached 

 at the time of laying. 



The Structure of the Egg at the Time of Laying. The 

 arrangement of structures in the egg at the time of laying 

 is shown in Figure 3. Most of the gross relationships are 

 already familiar because they appear so clearly in eggs which 

 have been boiled. If a newly laid egg is allowed to float free 

 in water until it comes to rest and is then opened by cutting 



nucleus of Pander _ blastoderm 



neck of latebra 



white yolk ^ T^fe^ ^ -- : ' '^'''^'.'^ ^ : - ;1 ''^J^^X 



S ^*^^*;^-.-:;. .-..:.-..... .^. \ N 



yellow yolk ' vitelline membrane 



PIG. 3. Diagram of the hen's egg in longitudinal section. {After Lillie.) 

 The relations of the various parts of the egg at the time of laying are indicated 



schematically. 



away the part of the shell which lies uppermost, a circular 

 whitish area will be seen to lie atop the yolk. In eggs which 

 have been fertilized this area is somewhat different in appear- 

 ance and noticeably larger than it is in unfertilized eggs. The 

 differences are due to the development which has taken place in 

 fertilized eggs during their passage through the oviduct. The 

 aggregation of cells which in fertilized eggs lies in this area is 

 known as the blastoderm. The structure of the blastoderm and 

 the manner in which it grows will be taken up in the next 

 chapter. 



Close examination of the yolk will show that it is not uniform 

 throughout either in color or in texture. Two kinds of yolk 



