182 COMPARATIVE ANATOMY 



do not differ in structure from ordinary colonies, but show a 

 marked differentiation in the course of their further history. 

 Certain of these colonies, which we may speak of as female 

 colonies, consist of thirty-two cells which we may call macro- 

 gametes, differing from the normal only in their somewhat 

 larger size and oval shape. The remaining colonies undergo 

 a process of cell-division leading- to the formation of male cells 

 or microgametes. Each individual cell of a male colony 

 divides to form a flat plate composed of sixteen or thirty-two 

 yellow cells which become rounded, and secrete an external 

 gelatinous wall round the plate. Gradually these cells become 

 elongated and spindle-shaped, they develop each a pair of 

 flagella at one end, and may now be called microgametes. 

 All the flagella of the cells composing a plate are turned in the 

 same direction, and eventually the composite plate moves about 

 by the action of its flagella, bursts out of the parent envelope, 

 and swims freely in the water. Meanwhile the female colonies 

 have come to rest (though their flagella remain attached and 

 execute slow waving movements), and their colonial envelopes 

 swell up and become gelatinised. When a free swimming plate 

 of microgametes meets a female colony in this condition it 

 attaches itself to it, the plate is resolved into its component 

 microgametes, and these bore their way into the gelatinised cyst- 

 wall of the female colony. Each microgamete works its way 

 towards a macrogamete and conjugates with it, the two fusing 

 to form a zygote which immediately becomes enclosed in a 

 thick cyst-wall. The further development of the zygote has 

 not been followed, but it probably does not differ much from 

 that of the zygotes of Pandorina. 



In both Pandorina and Eudorina all the cells composing the 

 colonies take their share in the reproductive processes, but 

 in the latter there is a differentiation into male and female 

 colonies, into microgametes and macrogametes, which, at the 

 most, is only obscurely indicated in the former species. But 

 in Volvox we find that a considerable advance has been 

 made, for the cells composing a colony are differentiated into 

 those which are nutritive but not reproductive, and those 

 which are reproductive but not nutritive, or, as it is said, into 

 somatic and generative cells. Volvox globator is tolerably 

 common in ponds and ditches. It has the form of a hollow 

 sphere of considerable size as compared to Pandorina and 



