1 84 DIVERSIONS OF A NATURALIST 



mating cells, like those regularly produced by the massive 

 plants and animals which are built up by hundreds of 

 thousands of "cells" or protoplasmic units conjoined 

 and performing different services for the common life. 

 Volvox is one of those simple aquatic organisms which 

 is not a single cell but a group of many cells (some 

 hundred) hanging together — in this case so as to form a 

 hollow sphere. All the cells of an individual sphere are 

 alike, and have originated by division from one first cell. 

 When the " breeding season " arrives one or two cells of 

 the sphere increase in bulk^they become " large mating 

 cells " — in fact, egg cells. At the same time one or two 

 divide (without separating), so as to form packets of 

 minute oblong cells with vibrating tails. These are 

 " small maters," or " spermatozoa." When ripe they 

 separate and swim away to fertilize — that is to say, to 

 fuse with — the large " mating cells " or egg^ cells of other 

 Volvox spheres. Such a Volvox sphere as I have 

 described is " bi-sexual " : it produces both large and 

 small mating cells, both male and female reproductive 

 cells. But sometimes we find that a number of Volvox 

 spheres produce only large mating cells by the swelling 

 up of one or two of their constituent cells. They are, 

 in fact, female Volvox spheres. And other Volvox 

 spheres produce only packets of small mating cells by 

 the splitting and change of one or two of their constituent 

 cells. They are male Volvox spheres. 



When we now look at the higher plants and animals 

 formed of aggregations of innumerable cells (all derived 

 from the division of a first cell — an embyro cell or 

 fertilized egg cell) we find that amongst the mass of 

 variously shaped cells forming the " tissues '' of these 

 higher organisms some are set apart even in early growth 

 as " mating cells " (gametes or reproductive cells). 



