225

ON THE SPHÆROSIRA VOLVOX OF EHRENBERG.

BY PROFESSOR WILLIAMSON, F.R.S.

[PLATE LXI.]

HEN the well-known aphorism of omne vivum ex ovo was

it was thought, by many advocates of what has long been called the cell theory, that physiologists had reached the most elementary form of organisation. But how far this is from being true is shown by the vast advance in knowledge of the subject which has taken place during the last twenty years. We have learned that what was thought to be the all-important cell is now but a comparatively insignificant product of the true generating organism; an incidental portion which may or may not be present, but the presence or absence of which has little, if any, influence upon the active protoplasmic element in which resides the true life, and by which is fulfilled the important functions of growth and reproduction. The outer cell membrane resolves itself mainly into a mere framework, secreted by the active protoplasm, and designed to provide a suitable protective chamber in which the chemical and physiological operations of the protoplasm can be conducted without disturbance from external forces. Beyond this protective function, it is doubtful if the cell-membrane fulfils any purpose save that of the membranous analyser regulating some of the osmotic phenomena of which all cells are the seat. The true phenomena of cell-life reside in the protoplasm, and if there is any truth in the doctrine of pangenesis, we may yet have to fall back upon the minute granules which abound in protoplasm as the true reproductive germs, endowed with a self-sustained vitality, and capable, under certain circumstances, of an independent life and action.

Since physiologists arrived at these conclusions many of the difficulties attending the study of the microscopic algæ, once so formidable, have disappeared like the dew before the

morning sun. Facts which appeared to be anomalous are now arranged in their proper places. The action of the protoplasm, independent of an investing cell-wall, can be studied in a thousand forms, and every such study is replete with interest. But in its progress this enquiry has developed a cognate one, and demonstrated that in many of the varied forms assumed by objects once thought to be distinct from each other, we have but so many manifestations of vital phenomena in individual species. Hence, rapid as has been the increase in the number of recorded specific forms of plants and animals since the publication of Ehrenberg's great work, it is questionable whether that increase has not been more than neutralised by the synthetic process by which it has been counterbalanced.

This synthetic aggregation of forms has been especially remarkable in the case of the Volvocina. That Volvox globator, aureus and stellatus, are but modified forms of the first-named species, is now an almost universally recognised truth. More difference of opinion, however, exists in reference to the curious little organism named by Ehrenberg Sphærosira Volvox. Towards the middle of April in the present year the Volvox globator appeared in marvellous abundance in a pond near my present abode, and amongst the common forms were myriads of Volvox aureus, and a considerable number of the Sphærosira. About the 10th of May some cold, wet weather arrived, and the whole stock disappeared from the pond; whilst of those which I was preserving at home no trace now (May 7th) exists, save vast numbers of the spores of Volvox aureus, which remain quiescent at the bottom of the glass jar in which they were contained. I availed myself of the opportunity thus afforded me of subjecting the Sphærosira to a rigid examination. The specimens were about the same size as the ordinary Volvox, but were more egg-shaped and rather longer in one direction than in the opposite one. This peculiarity, however, I found to be shared by many of the specimens of the common type of Volvox found in the same pool, though in a less marked degree. As is characteristic of the form under consideration, the separate masses of protoplasm scattered over the sphere varied in size; most of those at one end of the organism (fig. 1, a) being small, and the greater number of them uniform in size, being altogether undistinguishable from those of the associated Volvox. It has been affirmed, by Ehrenberg, that a distinction exists between the Sphærosira and the Volvox globator. He says that in the former each protoplasm is supplied with one cilium, whilst in the latter there are two. But I am convinced that this is not the case. True, I only found one cilium attached to each gemmule of Sphærosira, but I also found this

to be the case with nearly the whole of the individuals of Volvox globator in the same pond, demonstrating that, even in the latter, the number of the cilia is not a constant element, and that, consequently, it cannot be relied upon even as a specific, much less as a genuine, ground of distinction. This fact also explains the discrepancies hitherto existing in the observations of Ehrenberg, Dugardin, and Busk. Ehrenberg, as we have seen, separated Sphærosira from Volvox, on the ground that the former had one cilium to each gemmule, or gonidium, whilst the latter had two; but the other two observers just named deny the correctness of the statement on the ground that two cilia exist equally in both the forms. It is now evident that the number of these appendages varies both in Volvox and in Sphærosira. As we approach the opposite end of the sphere (fig. 1, b), we find that some of the gemmules begin to increase in size, the enlarged ones being apparently distributed without any special order or arrangement amongst those which remain comparatively unaltered. But as we advance, we find that nearly all the gemmules are either changed or changing into large compound masses these enlarged clusters of protoplasms occupying something like two-thirds of the entire sphere. On watching their development, we discover that they grow in precisely the same way as I, some years ago, demonstrated to be the case in Volvox globator. At first, one of the ordinary protoplasmswhich, by the way, have been variously designated by the terms gemmules, gonidia, and ciliated zoospores becomes slightly enlarged (fig. 2). It then appears as a green mass containing a few large and distinct granules in the midst of numerous very minute ones, diffused through the protoplasmic base. This mass soon divides by fission into two (fig. 3), and this again successively into four (fig. 4), eight, sixteen (fig. 5), and thirty-two (fig. 6), beyond which the fission was not carried in any of the examples which I examined. It thus appears that five successive acts of fissiparous segmentation took place, every one of the enlarged protoplasms undergoing division at each repetition of the process. So far, all this corresponds with the ordinary way in which the gonidia develop into the young spheres that give such beauty to the Volvox globator; yet a marked difference is seen in the results. In the Volvox, these successive divisions end in the formation of a small sphere-a miniature representative of the parent globe-but in Sphærosira the product is a flat disk, in which the aggregated and elongated protoplasms are arranged vertically to its surface (fig. 7), constituting an organism which, in a free state, closely resembles those types which Ehrenberg has elevated into genera under the names of Euroglena, Syncrypta, Uvella, and