GREEN-FLIES AND PLANT-LICE 



329 



" eggs " are merely shells and packing to hold and protect 

 this all-important egg-cell. 



Every individual flower, tree, insect, snail, fish, and 

 man started as a single egg-cell, 

 which became detached from the 

 mother's body. Take the case of 

 a common marine animal, the 

 star-fish. At the breeding season, 

 early in the year, the female star- 

 fish discharges thousands of these 

 egg - cells into the sea - water. 

 Each floats separately in a delicate 

 case of its own. Before any one 

 of those floating egg-cells can 

 commence to divide so as to 

 build up a new mass of cells 

 a new young star-fish it must 

 undergo the process of " fertilisa- 

 tion." That is to say, its sub- 

 stance must fuse with that of 

 a " sperm-cell." These " sperm- 

 cells " are discharged into the 

 sea-water in countless thousands 

 by the male star-fishes. They 

 are excessively minute, actively 



jling threads, swollen out at 



FIG. 61. A single egg-tube 

 or ovarian tube (usually there 

 are many) of an insect. 

 The youngest and smallest 

 eggs are at the narrow end. 

 o o are larger egg-cells with 

 a striated shell or envelope ; 

 g, nucleus of the egg-cell. 

 The unshaded egg is one 

 grown to full size, and in the 

 parthenogenetic aphis would 

 develop where it is without 

 fertilisation into a young 

 aphis. 



one end to form a little knob, the 

 " nucleus " of the sperm-cell (see 

 p. 134 for figures of the sperma- 

 tozoa, and eggs of the oyster). 

 The water is rendered cloudy by 



the abundance of these microscopic filaments, which are 

 called "spermatozoa." One sperm-cell, or spermatozoon 

 comes into contact, in the sea-water, with each of the dis- 

 charged floating egg-cells. It burrows into it and fuses or 



