VOLVOCINE^E. 57 



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 Volvox is usually about 1-60", and individuals 

 are to be foucd in each colony varying from this down to about 1-80". 

 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 ' gotiidia,' 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 sphasrical 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 23, Fig. 6.) 



" 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 reagents 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. 



" 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 

 gonidinm 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 23, 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 22, Figs. 1 and 3), of such tenuity, indeed, as to be frequently 

 invisible ; but as the use of certain reagents often brings these lines 

 into view where it had been previously impossible to detect them, and 

 as they may be sometimes discerned for an instant when the eye is 

 applied fresh and unfatigued to the microscope where even a moment 

 later they seem to be absent, it may be assumed that the structure is 

 universal, though often far too subtle to be detected. It is needless to 

 say that no skill of the draughtsman can even suggest its infinite deli- 

 cacy, while tbe figures given in books, not excepting the beautiful 

 drawings in Ehrenberg's ' Infusionsthierchen,' exaggerate the strength 

 of the connecting lines to tbe extent of grossly caricaturing the extreme 

 fineness of Nature's own handiwork. 



" To return to the gonidia and their history. A certain number of 

 these in each individual are selected to produce a group of young Vol- 

 voces within the parent sphere. The books fix this number as usually 

 four or eight ; but out of twenty-five individuals now in the field of my 

 microscope I find only three containing four incipient spheres of tbe 

 second generation, while only one contains eight, and there are four 

 containing five, six with six, ten with seven, and one with nine such 

 progeny. Almost every Volvox, when first discharged from the parent 



