The definite form of the Oscillariœ, and the processes attending their growth, as regards outward appearances, are well known : their tænia-like, jointed structure ; their contents composed of phycochrom ; their motility; the frequently fringed points of the filaments ; the continual subdivision into two of their joints ; their rapid growth ; their spreading, &c. Premising that all this is known, I shall consider this definite form as the starting-point of their developmental processes, and proceed to indicate the relations of the other forms to it.

I shall select as an example the Oscillaria tenuis, Ktz., as being that most abundantly met with, and affording subjects of observation under all sorts of pseudo-forms (Leptothrix, Phormidum, Symploca, &c.).

As they become older, the filaments of O-tenuis, at first bluish-green, assume a more yellow-green colour, their contents acquiring pretty nearly the hue of those of the Ulotrichœ. The filaments break up into perfectly-distinct joints, which, at first urceolate, soon become spherical. The minute yellowish-green gonidia thus arising, gradually increase in size, become motile, and present in all respects the aspect of Chlamydomonas. They move by means of delicate cilia, and present in the interior, usually in the centre, a clearer green spot, a sort of vacuole, as I believe, which, if I do not err, has been taken by others for an amylaceous granule. The contents of the gonidia are still finely granular, and of a yellowish-green colour.

These minute Chlamydomonades (as I shall term them, since no one has pointed out any morphological distinction) gradually enlarge ; a red eye-point becomes visible in them, and, presenting a thousand intermediate forms, they grow into perfect Euglenœ. The minute vacuole visible in them from the first, is, in all cases, still recognisable as a larger, clearer space in the mature Euglena, when the latter is extended. The finely-granular contents have become coarsely granular, the eye-point [perhaps the first indication of a reflecting spherule, having no further organization, for the purpose of conducting the luminous rays, without the intervention of any other medium than the fluid of the body of the Euglena to the perception of the individual ; thence, also, the red colour, complementary to green; the eye-point in Euglena sanguinea, on the other hand, is green !] has become larger, and of a deeper colour, the filaments much elongated, and the movements more suitable to the now extended shape of the body.

The uniform life of the Euglena, prolonged only by endosmosis, terminates, as regards the individual, after repeated division, in the quiescent or “protococcus-condition,” as it is termed, with which we are already acquainted from the observations of numerous zoologists and botanists (vide, among others, Cohn, on Stephanosphera). In this condition the Euglena constitutes a large motionless spore, or protococcus-like globule, in which the eye-point gradually disappears ; the gelatinous envelope surrounding, at a short distance, a large number of green gonidia, which had constituted the granular substance of the mobile Euglena.

After this prolonged quiescent condition, the common Euglena-envelope ultimately dissolves, and the gonidia escape, either singly, or still connected into aggregate masses, in the form of motile corpuscles (the microgonidia of authors). If a number of these remain conjoined, and move about with a rowing kind of movement, their locomotion being governed by a common spontaneity, they represent a volvox-like colony, which, perhaps, may even have been described as Volvox by authors.

The microgonidia of the Euglena, like those of all the algæ hitherto examined by me, are the motile parent-cells of extra-ordinarily-minute spiral filaments. They are, at first, green, gradually becoming pellucid, exactly like the spermatospheres of Spirogyra, presenting a monadiform aspect. A peculiar appearance arises when, in one of these aggregations of micro-gonidia, many remain green, whilst the others have already become clear as water, the mass then presenting, in fact, the aspect of being composed of two kinds of animalcules. Such or similar conditions would represent several species of the supposed genus Uuella (atomus, glaucoma, bodo, &c.)

Each ultimately colourless microgonidium, then, by the dissolution of its minute gelatinous envelope, discharges a small motile-spiral filament. In these, we have not the large spiral filaments of the Charœ, Equisetáceœ, and Ferns. Although, in Selaginella, these filaments are excessively minute, and visible only to the closest scrutiny (vide Hofmeister’s fig. in his ‘ Comparative Researches,’ Tab. xxvi., fig. 3), still in Oscillaria tenuis they are, perhaps, yet more delicate. This investigation is, of course, one of the most difficult nature, and demands the most acute vision.

These spiral filaments in the Oscillarice do not appear to be destined for the purposes of impregnation, for they gradually increase in length and thickness, soon exhibiting innumerable spiral turns, and between them in the latter condition, and the finest spermatic filaments, a thousand different transitionary forms are met with. From the spirillna-like condition, they pass, by an increase in length and a continual spiral movement, into a spirulina-like form. Finally, when their motile faculty has become weakened, they affix themselves by one extremity to any near, larger object (for instance, Conferva-filaments, &c.), whilst the other extremity continues to move about with a creeping motion,—the peculiar Oscillarian movement,—in performing which a young filament frequently returns to the spiral. The last-described condition constitutes the Leptothrix of authors. The filaments now gradually become thicker, and though, at first, of the lightest emerald green, they gradually assume a deeper and deeper tint. The first indications of articulation are perceptible in them, until at last a young Oscilldtoria is, again perfected.

The extremities of the young Oscillariœ, as is well known, are fringed with hairs : this may be an indication of the existence of cilia, previously invisible, on the head-end of the filament, which end has now become the point of the Oscillaria. But in young plants it is not merely one or a few apical cells that are so fringed, for I have, in filaments thus furnished, not unfrequently noticed them fringed for the length of, perhaps, 3050 joints. This circumstance recals to mind the long-ciliated spermatozoa of the Ferns and Equisetœ. This investigation will also afford ground for many important conclusions with respect to the motion of the Oscillariœ.

I shall return to many peculiarities of the Euglena in another place, in speaking of other Nostochineæ, and will here merely state that I first remarked their plant-like connexion in the Rimtlariœ, in the gelatinous substance of which, when the Rivularia was mature, I al ways met with these bodies.

Much time and continued study, together with a fortunate conjunction of circumstances and ingenious combinations, will be requisite to follow the filaments here described by me through the labyrinth of forms of algœ, with their heterogeneous generations, if it be wished to apply this idea throughout that class of plants.