Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/888

8G4 PROTOZOA [CILIATA. nuclei of tissue cells (compare Fig. I. ami Fig. XXV. 9, 11, 17). The fibrilla, 1 were supposed to be spermatozoids, and tliis erroneous view was confirmed by the observation of rod-like Bacteria (Schizomycetes) which in some instances infest the deeper proto plasm of large Ciliata. The true history of the changes which occur in the nuclei of conjugating Ciliata lias been determined by Biitschli (74) in sonic typical instances, but the matter is by no means completely under- sH$>tL. v v.The phenomena present very great obstacles to satis- fa -y examination on account of their not recurring very fre- <]id*. T.- arid passing very rapidly from one phase to another. Theyiarfve not been closely observed in a sufficiently varied number of genera to warrant a secure generalization. The follow ing scheme of the changes passed through by the nuclei must be regarded as necessarily referring to only a few of the larger Heterotricha, Holotricha, and Hypotricha, and is only probably true in so far as details are concerned, even for them. It is at the same time certain that some such series of changes occurs in all Ciliata as the sequence of conjugation. In most of the Ciliata by the side of the large oblong nucleus is a second smaller body (or even two such bodies) which has been very objectionably termed the nucleolus (Fig. XXV. 8), but is better called the " paranucleus" since it has nothing to do with the nucle olus of a typical tissue-cell. When conjugation occurs and a "syzvgium" is formed, both nucleus and paranucleus in each con jugated animal elongate and show tibrillar structure (Fig. XXV. 10). Each nucleus and paranucleus now divides into two, so that we get two nuclei and two paranuelei in each animal. Elongation and fibrillation are then exhibited by each of these new elements and subsequently fission, so that we get four nuclei and four para- nuclei in each animal (11, 12). The fragments of the original nucleus (marked N in the figures) now become more dispersed and broken into further irregular fragments. Possibly some of them are ejected (so-called "cell excrement") ; possibly some pass over from one animal to the other. Two of the pieces of the four-times- divided paranucleus now reunite (Fig. XXV. 13), and form a largish body which is the new nucleus. The remaining fragments of paranucleus and the broken down nucleus now gradually dis appear, and probably as a remnant of them we get finally a few cor puscles which unite to form the new paranucleus (14, 15). The conjugated animals which have separated from one another before the later stages of this process are thus reconstituted as normal Ciliata, each with its nucleus and paranucleus. They take food and divide by binary fission until a new period of conjugation arrives, when the same history is supposed to recur. The significance of the phenomena is entirely obscure. It is not known why there should be a paranucleus or what it may correspond to in other cells whether it is to be regarded simply as a second nucleus or as a structurally and locally differentiated part of an ordinary cell-nucleus, the nucleus and the paranucleus together being the complete equivalent of such an ordinary nucleus. An attempt has been made to draw a parallel between this process and the essential features of the process of fertilization (fusion of the spermatic and ovicell nuclei) in higher animals ; but it is the fact that concerning neither of the phenomena compared have we as yet sufficiently detailed knowledge to enable us to judge conclusively as to how far any comparison is possible. Whilst there is no doubt as to the temporary fusion and admixture of the protoplasm of the conjugating Ciliata, it does not appear to be established that there is any transference of nuclear or paranuclear matter from one indi vidual to the other in the form of solid formed particles. _ Conjugation resulting merely in rejuvenescence and ordinary fis sile activity is observed in many Flagellata as well as in the Ciliata. A noteworthy variation of the process of binary fission occurring in the parasite Opalina deserves distinct notice here, since it is inter mediate in character between ordinary binary fission and that multiple fission which so commonly in Protozoa is known as spore- formation. In Opalina (Fig. XXIV. 4) the nucleus divides as the animal grows ; and we find a great number of regularly disposed separate nuclei in its protoplasm. (The nuclei of many other Ciliata have recently been shown to exhibit extraordinary branched and even "fragmented "forms; compare Fig. XXIV. 2.) At a certain stage of growth binary fission of the whole animal sets in, and growth ceases. Consequently the products of fission become smaller and smaller (Fig. XXIV. 6). At last the fragments contain each but two, three, or four nuclei. Each fragment now becomes encased m a spherical cyst (Fig. XXIV. 7). If this process had occurred rapidly, we should have had a uninucleate Opalina breaking up at once into fragments (as a Gregarina does), each fragment being a spore and enclosing itself in a spore-case. The Opalina ranarnm lives in the rectum of the Frog, and the encysted spores are formed in the early part of the year. They pass out into the water and undergo no change unless swallowed by a Tadpole, in the intestine of which they forthwith develop. From each spore- se escapes a uninucleate embryo (Fig. XXIV. 8), which absorbs nourishment and grows. As it grows its nucleus divides, and so the large multinucleate form from which we started is reattained. This history has important bearingc, not only on the nature of sporulation, but also on the question of the significance of the multinucleate condition of cells. Here it would seem that the formation of many nuclei is merely an anticipation of the retarded fissive process. It is questionable how far we are justified in closely associating Opalina, in view of its peculiar nuclei, with the other Ciliata. It seems certain that the worm-parasites sometimes called Opalina;, but more correctly Anaplophrya, &c., have no special affinity with the true Opalina. They not only differ from it in having one large nucleus, but in having numerous very active contractile vacuolcs (75). Recently it has been shown, more especially by Gruber (84), that many Ciliata are multinucleate, and do not possess merely a single nucleus and a paranucleus. In Oxytricha the nuclei are largo and numerous (about forty), scattered through the protoplasm, whilst in other cases the nucleus is so finely divided as to appear like a powder or dust diffused uniformly through the medullary proto plasm (Trachelocerca, Choenia). Carmine staining, after treatment with absolute alcohol, has led to this remarkable discovery. The condition described by Foettinger (85) in his Opalinopsis (Fig. XXIV. 1, 2) is an example of this pulverization of the nucleus. The condition of pulverization had led in some cases to a total failure to detect any nucleus in the living animal, and it was only by the use of reagents that the actual state of the case Avas revealed. Curiously enough, the pulverized nucleus appears periodically to form itself by a union of the scattered particles into one solid nucleus just before binary fission of the animal takes place ; and on the completion of fission the nuclei in the two new individuals break up into little fragments as before. The significance of this observation in relation to the explanation of the proceedings of the nuclei during conjugation cannot be overlooked. It also leads to the suggestion that the animal cell may at one time in the history of evolution have possessed not a single solid nucleus but a finely n> Secular powder of ehromatin- substance scattered uniformly through its protoplasm, as we find actually in the living Trachelo cerca. Some of the Ciliata (notably the common Vorticella;) have been observed to enclose themselves in cysts ; but it does not appear that these are anything more than " hypnocysts " from which the aninial emerges unchanged after a period of drought or deficiency of food. At the same time there are observations which seem to indicate that in some instances a process of spore-formation may occur within such cysts (76). The differentiation of the protoplasm into cortical and medul lary substance is very strongly marked in the larger Ciliatn. The food-particle is carried down the gullet by ciliary currents and is forced together with an adherent drop of water into the medullary protoplasm. Here a slow rotation of the successively formed food-vacuoles is observed (Fig. XXV. 2, I, m, n, o), the water being gradually removed as the vacuole advances in position. It was the presence of numerous successively formed vacuoles which led Ehrenberg to apply to the Ciliata the not altogether inappro priate name " Polygastrica. " The chemistry of -the digestive pro cess has not been successfully studied, but A. G. Bourne (8) has shown that, when particles stained with water-soluble anilin blue are introduced as food into a Vorticella, the colouring matter is rapidly excreted by the contractile vacuole in a somewhat concen trated condition. The differentiation of the protoplasm of Ciliata in some special cases as "muscular" fibre cannot be denied. The contractile filament in the stalk of Vorticella is a muscular fibre and not simple undifferentiated contractile protoplasm ; that is to say, its change ot dimensions is definite and recurrent, and is not rhythmic, as is the flexion of a cilium. (Perhaps in ultimate analysis it is impossible to draw a sharp line between the contraction of one side of a cilium which causes its flexion and the rhythmical contraction of some muscular fibres.) The movements of the so-called " setai " of the Hypotricha are also entitled to be called "muscular," as are also the general contractile movements of the cortical substance of large Ciliata. Haeckel (77) has endeavoured to distinguish various layers in the cortical substance; but, whilst admitting that, as in the Gregarinse, there is sometimes a distinct fibrillation of parts of this layer, we cannot assent to the general distinction of a " myophane" layer as a component of the cortical substance. Beneath the very delicate cuticle which, as a mere superficial pellicle of extreme tenuity, appears to exist in all Ciliata we frequently find a layer of minute oval sacs which contain a spiral thread ; the threads are everted from the sacs when irritant reagents are applied to the animal (Fig. XXV. 2, g, It). These were discovered by-Allman (78), and by him were termed " tricho- cysts." They appear to be identical in structure and mode of formation with the nematocysts of the Cctlentera and Platyhelmia. Similar trichocysts (two only in number) are found in the spores of the Myxosporidia (see ante, page 855). The comparative forms of the nucleus and of the contractile

vacuoles, as well as of the general body-form, &c., of Ciliata may