234 STRUCTURE AND LIFE-HISTORY OF VOL VOX GLOEATOR. 



Professor Williamson describes the structure in question as a network 

 of lines dividing the whole surface into hexagons, in the centre of each 

 of which is seated one of the gonidia. 



The delicate " protoplasm-threads " proceeding from each of these 

 to its six surrounding neighbours never pass through the angles of the 

 hexagons, but always through the side of each hexagon to the next 

 gonidium. (Plate VII., Figs. 1, 3.) Hence it appears that " the points of 

 adhesion are chosen prior to the development of the outer cell- 

 membrane," in which light Williamson regards the hexagonal division. 

 In his specimens this structure was developed by immersion in glycerine 

 for some time. I have failed to obtain more than the faintest suggestion 

 of it by these means, but it is often brought out by the application of 

 aniline purple, as is also an important detail shown in drawings made 

 from his preparations, viz., that at the angles of the contiguous 

 hexagons there is sometimes a distinct doubling or separation of the 

 lines, whence he concludes that each side of the figure is really formed 

 by two delicate cell-walls in close juxtaposition, the duality of which 

 is only made evident by the action of re-agents. (Plate VII., Fig. 2.) He 

 regards the globe of Volvox as a " hollow vesicle, the walls of which 

 consist of numerous angular cells, filled with green endochrome, &c, the 

 intercellular spaces being more or less transparent," and the ciliated 

 zoospore as representing the endochrome of a cell having two walls, the 

 internal one being separated from the outer cell-wall, except at a few 

 points where it is retained in contact by the connecting filaments, and the 

 external one forming the hexagonal divisions on the surface. He further 

 holds that the periphery of the sphere, when seen in section, has an 

 appreciable thickness, its inner margin being definite and parallel to the 

 outer one ; and that the sides of the hexagons being continued downwards 

 through the thickness of the outer membrane, the appearance of all 

 these structures, if they could be seen simultaneously, would be that 

 shown in Plate VII., Fig. 5. 



Even in deeply-stained specimens I have never been able to detect 

 the existence of these hexagons as other than an entirely superficial 

 structure, and at present my impression is that the hexagonal structure 

 has a different significance. 



In the very early stage of Volvox-life, the embryo gonidia are encased 

 in a distinct transparent outer-sphere. (Plate VII., Figs. 4e and 6.) At 

 a later period, owing to the more rapid growth of the gonidia than of the 

 case, the latter closely invests the former, which are, in fact, embedded 

 in it. In the next stage, if not in the earlier condition, by the continued 

 growth of the gonidia at a greater rate than that of the containing sphere, 

 they are so closely appressed as to assume the hexagonal form, and the 

 interstices must, of necessity, consist of a thin film of the substance of 

 the containing spherical envelope, moulded, so to speak, into correspond- 

 ing forms. But now the diameter of the young Volvox, which is, by this 

 time, sent forth on its independont career, rapidly increases, the gonidia 

 assuming their spherical or pyriform shape, as their mutual pressure 

 diminishes, and being hourly separated by greater intervals. If now 



