THE PERPETUATION OF LIFE 333 



and cytoplasm so that each conjugating individual loses 

 its identity in the resulting product. In some of the one- 

 celled animals and plants the conjugating individuals, 

 instead of being of equal size, have become differentiated 

 into larger, relatively passive individuals on the one hand, 

 and smaller, more active ones on the other. In the colo- 

 nial flagellate Pandorina the conjugating cells are both 

 active and but slightly unequal in size, but in the related 

 form Eudorina and in Volvox the conjugating cells are 

 very dissimilar. One kind is large, spherical in form and 



A . _ _ 



FIG. 227. Fertilization of the egg. A, egg entered by a sperm cell, s; 

 B, the nucleus of the head of the sperm cell, s, enlarged and approaching 

 the nucleus of the egg, n ; C, sperm nucleus, 5 more enlarged and in contact 

 with the egg nucleus n; D, egg nucleus and sperm nucleus fused together. 



devoid of activity; while the other is small, with an ovoid 

 head and a lash-like tail by means of which it swims through 

 the water. One of the small active cells meets and fuses 

 with, or fertilizes, one of the large ones which, after going 

 into a resting stage, produces by repeated division a new 

 colony. 



In all the multicellular animals the sex cells are dif- 

 ferentiated into two very sharply contrasted types, the 

 relatively large and inactive ova, or egg cells, and the small 

 active spermatozoa, or sperm cells whose function it is to 

 meet and fertilize the eggs. The eggs of nearly all animals 

 require to be fertilized before they can develop, but in 

 exceptional cases eggs may develop without being fertilized. 

 This process which is called parthenogenesis (see page 35) 

 is well illustrated by the generation of the aphids or plant 



