42 STRUCTURE AKD LIFE-HISTORY OF VOLVOX GLOBATOR. 



dark back-ground illumination, and reveals details which are wholly 

 invisible without its use. 



The colour is, however, greedily absorbed by some of the materials 

 used by the microscopist, so that a judicious choice of these is necessary 

 to ensure success. Objects stained in this manner are, for instance, 

 rapidly bleached if mounted in gold size cells, and I have for the 

 present adopted zinc-white in its place. Among other re-agents which I 

 have used are eosin, iodine, iodised glycerine, carmine solution, 

 potassium permanganate, nitrate of silver, and other salts, some of 

 which bring into view various parts of the minute structure of plants ; 

 but aniline colours, applied with due precautious, produce the most 

 rapid and striking effect. 



Professor Williamson describes the structure in question as a network 

 of lines dividing the whole surface into liexagons, 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 iigure 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 

 e.rtei-nal 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. 



