KEPKODUCTION 



91 



called the male and the female. They give rise to cells, 

 which differ from each other very definitely. Those 

 formed in the female are large, round and have no power 

 of themselves to move. They are called egg cells, or ova. 

 The organ in which they are formed is the ovary. In the 

 male the cells are small, irregularly shaped and motile 

 (Fig. 54). They are called sper- 

 matozoa, or sperm cells, and the organ 

 in which they are formed is the 

 sperm ary. 



Attraction of the Sexual Cells. — 

 The egg cells have a chemical attrac- 

 tion for the sperm cells. By vibrat- 

 ing the long tail-like appendage, the 

 sperm cells move through the water 

 and approach the egg cells. One of 

 them punctures the wall of an egg 

 cell, and its nucleus, with the slight 

 amount of protoplasm surrounding 

 it, enters. The tail is left outside and 

 finally disintegrates. A single cell 

 then remains, containing two nuclei, 

 one belonging to itself, the other to a 

 cell of different character. These 

 nuclei fuse. The large cell with now 

 but a single nucleus has, like the one-celled animal after 

 conjugation, a new and increased power of division. 



Division of the Fertilized Egg. — It divides very 

 rapidly first into two cells, which remain attached to 

 each other, then into four, eight, sixteen, thirty-two 

 cells and so on, until a mass like a mulberry is formed 



Fig. 54. — Spermatozoa. 



A, of the nighthavvk ; 



B, of the green frog; 

 n, nucleus;^ m, middle 

 piece; s, tail. (After 

 Hertwig.) 



