STRUCTURE AND LIFE-HISTORY OF VOLVOX GLOBATOR. 87 



chiefly, no donbt, because they can only bo revealed by the careful use of 

 somewhat hi<:»h powers, assisted by the action of various re-agents. 



Findin<i that the descriptions given in ordinary text-books were 

 often too meagre to be of much value, and as often hopelessly obscure, I 

 was led to consult carefully the original memoirs of Colin, Busk, 

 Williamson, Ac, on this subject, and, wherever it was possible, to i-epeat 

 their observations — and having acquired by this means a far clearer idea 

 of the structure of Volvox than I had hitherto possessed, having also, 

 perhaps, added some few new facts to the common stock of such 

 knowledge, I have thought that a short summary of what is known 

 of its life-history might be of some use to my fellow-students. 



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 perfoctness, 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 iiincr 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 

 exti'emity of nearly equal radii. These green points are " fionidia," 

 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 gouidium 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 VII., 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 ^^eije-spoi," and all the eye-spots in an individual look, so 

 to speak, one way. (Plate VII., Fig. oa.) 



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

 parent point, from which i>roceed two cilia, several times as long as tlie 

 Touidium 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 



