212 STRUCTURE AND LIFE-HISTORY OF VOLVOX GLOBATOR. 



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

 c avity of the parent plant. (Plate VII. , Fig. 4c c'.) Repeated sub-division 

 now goes on rapidly, (Plate VII., Fig. \d d',) till the whole body assumes 

 a spherical 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 re-agents, 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 VII., 

 Fig. 4e e', and Fig. 6.) Finally, the young Volvox consists of a vast 

 number of deep green granules, closely packed together, and by mutual 

 pressure driven to assume a more or less distinctly hexagonal form, and 

 corresponding in number to the gonidia which are to stud its surface 

 when its growth is completed. (Plate VII., Fig. 6a.) Shortly hereafter, 

 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 that of the gonidia, which are now approaching 

 maturity. The interlacing connecting threads are developed simul- 

 taneously. (Plate VII., Fig. la.) During the whole process, the centre 

 of the young Volvox 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 

 closely 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 VEL, Fig. 6.) 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. (Plate VTL, Fig. 6.) 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 progressive 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 

 greater part of the time it is regularly in the direction indicated, and 

 the point of rupture of the spliere will he at its north pole. (Plate VII., 

 Fig. 6.) 



It is difficult to determine precisely how this rupture is accom- 

 plished, but I believe it to be by a special contraction of the walls of the 



