STRUCTrUE ANT) LIFE-IIISTORY OF VOLVOX (iLOKATOII. 43 



In the very early staj,'c of Volvox-lifc, the embryo {»oiii(lia are encased 

 in a distinct transparent outer-sphere. (Plate VII., Fifi;s. ic and G.) 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 gouidia at a (jrcatcr 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 independent 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 

 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 distended 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 necessarihj 

 assumed by a number of spherical bodies under mutual pressure, so the 

 most economical disposition of this particular part of theVolvox-structure 

 will necessitate its constant attenuation into hexagons of ever-increasing 

 delicacy. (Plate VII., Figs. 1, 3, 7.) If the process be continued long 

 enough, it may finalh' 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 

 "mucilaginous fluid." If a Volvox be ruptured under a cover-glass, and 

 aniline purple introduced by capillary attraction, the colour seems to be 

 for awhile 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 proceeds streaks or lines, 

 radiating more or less regularly from its soiith pole. (Plate VII., Fig. 6.) 

 This structure, to which I do not think attention has been hitherto 

 called, is also sometimes developed in deeply-stained specimens within 

 the slightly ruptured sphere, and seems to show that there is a denser 

 layer of thick matter, whatever its nature may be, disposed in a some- 

 what regular manner, being concentrated near the south pole of the axis 

 of rotation, whence it spreads over the inner surface in streaks resembling 

 the lines of longitude on a terrestrial globe. 



Both from its position and from the rapidity with which it is stained 

 by aniline purple — without which its existence is apparently absolutely 

 undemonstrable — (in which respect it is in marked contrast to the outer 

 cell-wall, which latter is only faintly tinted by somewhat prolonged 



