58 COCCOI'HYCE^E. 



sac, and possessing a diameter of about 1-170", already contains a 

 certain number of enlarged gonidia, destined in due time to become its 

 own progeny. Not only so, but long before its discharge, and while yet 

 it exists as a daughter-cell within the protecting cavity of the parent 

 generation, these selected gonidia are already visible as spots larger and 

 darker than their fellows. (Plate '22, Fig. l'.) 



" The history of these selected gonidia, as it may be traced in a 

 daughter-sphere recently cast foith to seek its fortunes in the world of 

 waters around it, is as follows : The enlarged gonidium is at first a flat, 

 thin circular disc, appressed to the internal surface of the sphere, and 

 being surrounded by eight of the ordinary zoospores, is derived from 

 the coalescence of the two central ones out of a group of ten. (Plate 

 23, Figs. 1, la.) Shortly, this disc assumes a more distinctly oval form, 

 with a slight constriction across its lesser diameter, in which stage it 

 often much resembles a young Cosmarium. (Plate 23, Figs. '2, 2a.) It 

 is soon seen to be clearly subdivided into four, and its thickness having 

 grown pari passu with its superficies, the group now protrudes into the 

 internal cavity of the parent-plant. (Plate 23, Figs. 3, 3a.) Repeated 

 subdivision now goes on rapidly (Plate 23, Figs. 4. 4), till the whole 

 body assumes a sphserical form, a distinct cell wall being at the same 

 time formed, which is revealed by careful illumination, and still more 

 clearly by the use of reagents, as a hyaline sphere concentric to and of 

 larger diameter than the green one within it, so that there appears to be 

 a clear space or ring between the two when seen in section. (Plate 22, 

 Fig. 5, 5a.) Finally, the young Volvox consists of a vast number of 

 deep green granules closely packed together, and by mutual pressure 

 driven to assnme a more or less distinctly hexagonal form, and corres- 

 ponding in number to the gouidia which are to stud its surface when its 

 growth is completed. Shortly hei'eafter, the whole organism continuing 

 to increase in size, clear spaces appear between the gonidia, showing 

 that the enlargement of the cell wall and its interspaces is outstripping 

 tli at of the gonidia, which are now approaching maturity. The inter- 

 lacing connecting threads are developed simultaneously. (Plate 22, Fig. 

 la.) During the whole process the centre of the young Yolvox spheres 

 continually recedes from the periphery of the parent, so that when the 

 group of young ones has attained the full development of which it is 

 capable in this stage they are often pretty cl sely packed in the internal 

 space, and sometimes even slightly deformed by mutual pressure, each 

 by this time closely resembling the parent in miniature, and already 

 containing enlarged gonidia of the third generation. (Plate 22, Fig. 1.) 

 By this time the clear space originally visible between the gonidia and 

 the cell wall has been obliterated, and the cilia may be seen protruding 

 through the latter. Some writers state that the daughter-cells rotate 

 at this period within the parent cavity. I have frequently seen them 

 oscillate so far in one direction and then back to their original position, 

 but have never observed a true rotatory motion. Finally, the young 

 Volvoces are liberated by the rupture of the parent sac, at a special 

 point, clearly marked out for this purpose in its structure. I have not 

 met with any observations on this point, but have fully convinced 

 myself that it may always be predicted at what point this rupture will 

 be effected. 



" The combined action of the pairs of cilia in which the gonidia 

 terminate is the actuating power whence proceed both the rotatory and 

 the progressive movement of Volvox, and these are both in a definite 

 direction. If an imaginary axis be drawn through the sphere, the pro- 

 gressive motion being, so to speak, from the north to the south pole of 

 that axis, the rotatory motion is usually from west to east, though not 

 always, being occasionally reversed for a few seconds ; but for the 



