PROTOZOA 837 does not constitute an essential difference. The fission of the Ciliate Protozoan, Opalina (see below Fig. XXIV. 4-8), is a step from the ordinary process of delayed binary divi sion towards spore-formation. In some Protozoa spores are produced after encystation by a perfectly regular process of cleavage (comparable to the cleavage of the egg-cell of Enterozoa) first two, then four, then eight, sixteen, and thirty-two fission products being the result (see Fig. XX. 24, 25, &c.). But more usually there is a hastening of the process, and in these cases it is by no means clear what part the parent cell-nucleus takes. An encysted Gregarina (or two conjugated Gregarinse) suddenly breaks up into a number of equal-sized spores, which do not increase in number by binary division and have not been formed by any such process. This multicentral segregation of the parent pro toplasm is a marked development of the phenomenon of sporulation and remote from ordinary cell-division. How it is related to ordinary cell-division is not known, inas much as the changes undergone by the nucleus in this rapid multicentral segregation of the parent protoplasm have not been determined. The spores of Protozoa may be naked or encased singly or in groups in little en velopes, usually of a firm horny substance (see Fig. XX. 23 to 26, and Fig. XXIV. 15 to 18). Whenever the whole or a part of a Protozoon cell divides rapidly into a number of equal-sized pieces which are simultane ously set free and are destined to reproduce the adult form, the term spore is applied to such pieces, but the details of their formation may vary and also those of their subsequent history. In typical cases each spore produced as the result of the fission of an encysted Protozoon (con jugated or single) has its own protective envelope, as in the Mycetozoa (Fig. III.) and the Sporozoa (Fig. XVIIL), from which the contained protoplasm escapes by " ger mination " as a naked corpuscle either flagellate or amoebi- form. In some terminologies the word " spore " is limited to such a " coated " spore, but usually the naked proto plasmic particles which issue from such " coated " spores, or are formed directly by the rapid fission of the parent Protozoon, are also called " spores." The former condition is distinguished as a "chlamydospore," whilst the latter are termed " gymnospores." Many Protozoa produce gymno- spores directly by the breaking up of their protoplasm, and these are either " flagellulae " (swarm-spores) or "amce- buke " (creeping spores). The production of coated spores is more usual among the lower plants than it is among Protozoa, but is nevertheless a characteristic feature of the Gregarinae (Sporozoa) and of the Mycetozoa. The term " gemma " or " bad-spore " is applied to cases, few in number, where (as in Acinetaria, Fig. XXVI. , Spiro- chona, Fig. XXIII. 10, and Reticularia, Fig. X. 8) the spores are gradually nipped off from the parent-cell one or more at a time. This process differs from ordinary cell-division only in the facts (1) that the products of division are of unequal size the parent-cell being distin guishable as the larger and more complete in structure, and (2) that usually the division is not binary, but more than one bud-spore is produced at a time. Whilst in the binary cell-division of the Protozoa the two products are usually complete in structure at the period of separation, spores and spore buds are not only of small size and therefore subject to growth before attaining the likeness of the parent, but they are also very often of simple and incomplete structure. The gap in this respect between the young spore and its parent necessarily varies according to the complexity of the parental form. In the case of the Eadiolaria, of the Gregarinoe, of Noctiluca, and of the Acinetaria, for instance, the spore has before it a considerable process of development in structure and not merely of growth, before attaining the adult characters. Hence there is a possible embryology of the Protozoa, to the study of which the same prin ciples are applicable as are recognized in the study of the embryology of Enterozoa. Embryonic forms of great sim plicity of structure, often devoid of nucleus, and consist ing of simple elongate particles of protoplasm, are hatched from the spore-cases of the Gregarinae (Fig. XVII. 13, 14) These gradually acquire a differentiated cortical protoplasm and a nucleus. A very large number of Gymnomyxa pro duce spores which are termed " monadiform," that is, have a single or sometimes two filaments of vibratile protoplasm extended from their otherwise structureless bodies. By the lashing of these flagella the spores (swarm-spores or zoospores) are propelled through the water. The resem blance of these monadiform young (best called " flagel- lulae ") to the adult forms known as Flagellata has led to the suggestion that we have in them a case of recapitula tive development, and that the ancestors of the Gymno myxa were Protozoa similar to the Flagellata. Again the Acinetaria produce spores which are uniformly clothed with numerous vibratile cilia (Fig. XXVI), although the adults are entirely devoid of such structures ; this is accounted for by the supposition that the Acinetaria have been developed from ancestors like the Ciliata, whose characters are thus perpetuated in their embryonic stages. There can be little doubt that these embryological sugges tions are on the whole justified, and that the nucleated Protozoa are the descendants of non-nucleated forms simi lar to the spores of Gymnomyxa and Sporozoa, whilst it seems also extremely probable that the ancestral Protozoa were neither exclusively amoeboid in the movement of their protoplasm nor provided with permanent vibratile filaments (flagella and cilia) ; they were neither Myxopods nor Mastigopods (to use the terms which have been intro duced to express this difference in the character of the locomotor processes), but the same individuals were capable of throwing out their protoplasm sometimes in the form of flowing lobes and networks, sometimes in the form of vibratile flagella. A few such undifferentiated forms exist at the present day among the Proteomyxa and in a little more advanced condition among the lowest Flagellata, e.g., Ciliophrys. Death. It results from the constitution of the Protc- zoon body as a single cell and its method of multiplication by fission that death has no place as a natural recurrent phenomenon among these organisms. Among the Entero zoa certain cells are separated from the rest of the consti tuent units of the body as egg- cells and sperm-cells ; these conjugate and continue to live, whilst the remaining cells, the mere carriers as it were of the immortal reproductive cells, die and disintegrate. There being no carrying cells which surround, feed, and nurse the reproductive cells of Protozoa, but the reproductive cell being itself and alone the individual Protozoon, there is nothing to die, nothing to be cast off by the reproductive cell when entering on a new career of fission. The bodies of the higher animals which die may from this point of view be regarded as something temporary and non-essential, destined merely to carry for a time, to nurse, and to nourish the more import ant and deathless fission-products of the unicellular egg. Some of these fission-products of the new individual de veloped from an egg-cell namely, the egg-cells and sperm- cells are as immortal as the unicellular Protozoon. This method of comparing the unicellular and the multicellular organism is exceedingly suggestive, and the conception we thus gain of the individuality of the Enterozoon throws light upon the phenomena of reproduction and heredity in those higher organisms.
Experiment and observation in this matter are extremely