PROTOZOA 303 



whole structure of which is framed so as only to divide 

 and develop after amphimixis, the power of developing 

 without union. Apart from these the importance of 

 amphimixis led to the disappearance of the asexual 

 germ-cells, and the animals were forced to effect 

 amphimixis if they wished to multiply. 



It will have already occurred to the reader that the 

 inactive sexual germ-cells of the volvox correspond to 

 the ova of the higher animals, and the active germ-cells 

 to the spermatozoa ; even in man the latter have retained 

 the lashes of their flagellate period. But, whereas in 

 the volvox the same individual produces both kinds of 

 germ-cells, as is the case with many other animals, which 

 we call hermaphrodites, we find most animals divided 

 into male and female, each with only one kind of germ- 

 cells. These are matured at special parts of the body, 

 namely, in the ovaries or testicles, as the case may be. 

 They have diverged more and more from each other in 

 structure. In this the chief principle was the division of 

 labour. The germ-cells had to find each other, and had 

 to provide a certain amount of nourishment for the 

 development of the young after combining. The first 

 function devolved on the spermatozoa, which are 

 endowed with mobility according to the requirements of 

 each species. As a rule they are equipped with mobile 

 lashes, and are produced in enormous quantities, which 

 is facilitated by their smallness. They can be small, 

 because they do not need to contribute any nourishment 

 for the new organism, but only the paternal characters 

 that are found in their nucleus. The food is provided 

 by the ovum, and so this may attain considerable 



