210 STRUCTURE AND LIFE-HISTORY OF VOL VOX GLOBATOR. 



It seems hardly necessary to describe the normal aspect of this 

 organism. Briefly, under a low power, it is seen to consist of a 

 spherical globe of mathematical perfectness, so transparent that, as it 

 glides along, any object over which it passes is clearly visible through 

 its vacant spaces, i.e., through such parts as are not occupied by the 

 structures presently to be noticed, while by focussing the binocular on 

 the lower half of the plant, the effect is obtained of looking into the 

 inside of a glass sphere of crystalline purity and of absolute symmetry. 

 The diameter of a full-grown individual is usually about 1-60", and 

 individuals are to be found in each colony varying from this down to 

 about 1-180". The inner surface of the sphere is studded at intervals 

 with dark green points, not disposed irregularly, but so arranged that 

 each is usually the centre of a group of six others, placed at the 

 extremity of nearly equal radii. These green points are " gonidia,'' 

 each probably endowed with the potentiality of becoming a perfect 

 Volvox, though only a certain number of them actually undergo that 

 sequence of changes which results in their becoming fresh individuals 

 resembling the parent sphere. 



Each gonidium is either spherical or pyriform, (in which case its 

 pointed end is directed outwards,) and contains, in its early stages at 

 any rate, one or more contractile vacuoles disposed among a mass of 

 granular endochrome, and stated by Busk to pulsate rhythmically once 

 in about forty seconds. (Plate "VTL, Fig. 5.) 



At this period are also to be seen in the body of the gonidium one, 

 two, or three — occasionally even more — brilliant colourless spots, from 

 one of which is probably derived a nucleus which can be detected by 

 the use of re-agents at a later period. 



There is also often lodged within the substance of the zoospore a 

 brown or red " eye-spot," and all the eye-spots in an individual look, so 

 to speak, one way. (Plate VLI., Fig. 5a.) 



The apex of each gonidium is more or less produced into a trans- 

 parent point, from which proceed two cilia, several times as long as the 

 gonidium itself, which pass through two minute pores in the outer 

 cell wall, and move freely in the surrounding water. I am fortunate 

 in having mounted a specimen of Volvox, in which these pairs of 

 foramina are clearly shown, and the regularity of their disposition at a 

 uniform angle to the equator of the sphere is striking. (Plate VII., Fig. 7.) 

 It is, of course, by the combined action of these numerous pairs of cilia 

 that the whole organism progresses. Of the direction of the resultant 

 motion we shall speak shortly. 



Viewing the surface of the sphere with its convexity presented to 

 the objective, we find, by very careful adjustment of light, that from 

 each gonidium there runs to each of the six surrounding ones a fine 

 thread, sometimes double, occasionally triple, always of extreme 

 tenuity, (Plate VII., Fig. 1 ;) of such tenuity, indeed, as to be frequently 

 invisible ; but as the use of certain re-agents often brings these lines 



