iv] THE SECOND GRADE 105 



where Yolvox has made the great advance, the cells 

 are not all alike. Most are of the type already seen 

 in Gonium and characteristic of the family ; these row 

 the colony through the water, steer it, and feed it. 

 Amongst them, in the hinder half of the sphere, 

 are larger cells, lacking flagella and eye-spot, and 

 connected by very numerous strands with their neigh- 

 bours, 



"Their oarsmen-brothers, by whose toil, safe fed 

 And guarded safe, they live a charmed life 

 Within their latticed crystal, peaceably." 



And what do they do in return? Now is discovered 

 the skeleton in the flagellated cells' cupboard they 

 cannot reproduce the colony. They are sterile, and 

 must leave reproduction to the big lazy-seeming cells 

 who are only lazy, however, because they must store 

 up food-materials to start the new colony fairly on its 

 way. They grow and grow, bulge inwards, and finally 

 come to float free in the centre space, where they still 

 grow, meanwhile dividing up into a number of cells. 

 In the end, they become perfect miniature colonies, 

 burst out of their parent and swim happily away. 



Yolvox is thus a real individual ; of the two kinds 

 of cells each has given up something the better to 

 fulfil its own special duty. There is division of 

 labour, and, from the point of view of the species, 

 each kind is meaningless apart from the other. 



The division of labour in Yolvox is that usually 



