VOLVOCINE^E. Cl 



only made evident by the action of reagents. (Plate 22, Fig. 2.) He 

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

 consist of numerous angular cells filled with green euclochrome, &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 appear- 

 ance of all these structures, if they could be seen simultaneously, would 

 be that shown in Plate 23, Fig. 6. 



" 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 23, Fig. 5, 5a.) 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 hex- 

 agonal form, and the interstices must of necessity consist of a thin film 

 of the substance of the containing sphserical envelope, moulded, so to 

 speak, into corresponding forms. But now the diameter of the young 

 Volvox, which is by this time sent forth on its independent career, 

 rapidly increases, the gonidia assuming their sphserical or pyriform 

 shape as their mutual pressure diminishes, and being hourly separated 

 by greater intervals. If, now, the actual formative matter of the sphere 

 receives no further or only a disproportionate increment, but is 

 gradually attenuated by continued expansion, as a soap bubble is dis- 

 tended by blowing into it, the hexagonal lines into which it has been 

 moulded by the previous mutual pressure of the embryo gonidia will be 

 gradually stretched in all directions into finer proportions; and just as 

 this figure is that which is necessarily assumed by a number of sphaerical 

 bodies under mutual pressure, so the most economical disposition of this 

 particular part of the Volvox-structure will necessitate its constant 

 attenuation into hexagons of ever-increasing delicacy. (Plate 22, Figs. 

 1,3; Plate 23, Fig. 7.) If the process be continued long enough, it 

 may finally result in the structure becoming too filmy to be detected by 

 any microscopical observation ; and it is worth noticing that it is 

 usually in spheres of small or medium diameter that the hexagonal 

 divisions can be developed, and not in those of the largest size. Such 

 appears to me at present to be the rationale of the formation of this 

 structure. 



" The internal cavity of the sphere is said to be filled with a * muci- 

 laginous fluid.' L If a Volvox be ruptured under a cover-glass, and aniline 

 purple introduced by capillary attraction, the colour seems to be for a 

 while repelled at that part which is in front of the rupture, and to flow 

 round it on either side. It is only after a considerable time that it 

 gradually penetrates this space, and brings out, by staining it of a deep 

 purple tint, a mass of hazy matter, from which proceed streaks or 

 lines radiating more or less regularly from its south pole. This structure, 

 to which I do not think attention has been hitherto called, is also some- 

 times developed in deeply stained specimens within the slightly 



K 



