PROTOZOA 



35 



nuclei of tissue cells (compare Fig. I. and Fig. XXV. 9, 11, 17). 

 The tibrilhe were supposed to lie apermatozoids, and this erroneous 

 view was confirmed by the observation of rod-like Bacteria 

 (Schizomycetes) which in some instances infest the deeper proto- 

 plasm of large L'iliata. 



The true history of the changes which occur in the nuclei of 

 conjugating Ciliata has been determined by Butschli (74) in some 

 typir.'il instances, but the matter is by no means completely under- 

 stood. The phenomena present very great obstacles to satis- 

 factory examination on account of their not recurring very fre- 

 quently and passing very rapidly from one phase to another. 

 They have 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 " parauucleus " since it has nothing to do with the nucle- 

 olus of a typical tissue-cell. When conjugation occurs and a 

 "syzygium" is formed, both nucleus and paranucleus in each con- 

 jugated animal elongate and show fibrillar structure (Fig. XXV. 

 10). Each nucleus and paranncleus now divides into two, so that 

 we get two nuclei and two paranuclei 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- 

 sive 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 

 in 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 ranarum 

 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- 

 case 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 bearings, not only on the nature of 

 sporulatiou, 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 Opalinre, 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 racuoles 

 (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 large 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 was 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 

 molecular powder of chromatin-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 animal 

 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 Ciliata. 

 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 aniliu 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 of 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 " setae " 

 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 Gregarinre, 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, h). 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 Ccelentera 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 



