AN INTRODUCTION TO ZOOLOGY 



times two may arise and complete their development side by side. 

 In most cases, however, when two or more eggs are contained in 

 one ovary, their adjacent walls dissolve and one of the nuclei sur- 

 vives while the others disintegrate. As the ovum grows it becomes 



FIG. 63. An ovary (A), and mature egg (B) of 

 Hydra, a, egg nucleus ; ps., pseudopodium 

 of young egg ; em, egg membrane. (After 

 Tannreuther in Biol. Bui.) 



ameboid in shape, showing distinct pseudopodia (Fig. 63, A, p.s.) ; 

 these are drawn in when it has reached its full size. The egg is 

 now nearly spherical, and is surrounded by a single layer of ecto- 

 derm cells (Fig. 54, m.e.). Maturation then takes place. Two 

 polar bodies (p.b.) are formed, the first being larger than the sec- 

 ond. During maturation the number of chromosomes is reduced 

 from the somatic number, twelve, to six. This occurs at the end 

 of the growth period. Now an opening appears in the ectoderm 

 and the egg is forced out, finally becoming free on all sides except 

 where attached to the animal (Fig. 63, B). 



