Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/856

Rh 832 PROTOZOA ing Protozoa from Protophyta, and the insuperable diffi culty in really accomplishing the feat satisfactorily, has led at various times to the suggestion that the effort should be abandoned and a group constituted confessedly containing both unicellular plants and unicellular animals and those organisms which may be one or the other. Haeckel has proposed to call this group the Protista (I). 1 On the whole, it is more satisfactory to make the attempt to dis criminate those unicellular forms which belong to the animal line of descent from those belonging to the veget able line. It is, after all, not a matter of much conse quence if the botanist should mistakenly claim a few Protozoa as plants and the zoologist a few Protophyta as animals. The evil which we have to avoid is that some small group of unattractive character should be rejected both by botanist and zoologist and thus our knowledge of it should unduly lag. Bearing this in mind the zoologist should accord recognition as Protozoa to as wide a range of unicellular organisms as he can without doing violence to his conceptions of probability. A very interesting and very difficult subject of speculation forces itself on our attention when we attempt to draw the line between the lowest plants and the lowest animals, and even conies again before us when we pass in review the different forms of Protozoa. That subject is the nature of the first protoplasm which was evolved from not-living matter on the earth s surface. Was that first protoplasm more like animal or more like vegetable proto plasm as we know it to-day ? By what steps was it brought into existence ? Briefly stated the present writer s view is that the earliest proto plasm did not possess chlorophyll and therefore did not possess the power of feeding on carbonic acid. A conceivable state of things is that a vast amount of albuminoids and other such compounds had been brought into existence by those processes which cul minated in the development of the first protoplasm, and it seems therefore likely enough that the first protoplasm fed upon these antecedent steps in its own evolution just as animals feed on organic compounds at the present day, more especially as the large creeping plasmodia of some Mycetozoa feed on vegetable refuse. It indeed seems not at all improbable that, apart from their elaborate fructification, the Mycetozoa represent more closely than any other living forms the original ancestors of the whole organic world. At subsequent stages in the history of this archaic living matter chlorophyll was evolved and the power of taking carbon from carbonic acid. The &quot;green&quot; plants were rendered possible by the evolution of chlorophyll, but through what ancestral forms they took origin or whether more than once, i.e., by more than one branch, it is difficult even to guess. The green Flagellate Pro tozoa (Volvocinefe) certainly furnish a connecting point by which it is possible to link on the pedigree of green plants to the primi tive protoplasm ; it is noteworthy that they cannot be considered as very primitive and are indeed highly specialized forms as com pared with the naked protoplasm of the Mycetozoon s plasmodium. Thus then we are led to entertain the paradox that though the animal is dependent on the plant for its food yet the animal preceded the plant in evolution, and we look among the lower Protozoa and not among the lower Protophyta for the nearest representatives of that first protoplasm which was the result of a long and gradual evolution of chemical structure and the starting point of the development of organic form. The Protozoan Cell-Individual compared with the Typical Cell of Animal and Vegetable Tissues. MORPHOLOGY. The Protozoon individual is a single corpuscle of proto plasm, varying in size when adult from less than the TTnnjth of an inch in diameter (some Sporozoa and Flagel- lata) up to a diameter of an inch (Nummulites), and even much larger size in the plasmodia of Mycetozoa. The sub stance of the Protozoa exhibits the same general properties irritability, movement, assimilation, growth, and division and the same irremediablechemical alteration as the result of exposure to a moderate heat, which are observed in the protoplasm constituting the corpuscles known as cells which build up the tissues of the larger animals and 1 These numbers refer to the bibliography at p. 866. plants. There is therefore no longer any occasion to make use of the word &quot; sarcode &quot; which before this identity was established was very usefully applied by Dujardin (2) to the substance which mainly forms the bodies of the Protozoa. Like the protoplasm which constitutes the &quot; cells &quot; of the Enterozoa and of the higher plants, that of the Protozoon body is capable of producing, by chemical processes which take place in its substance (over and above those related merely to its nutrition), a variety of distinct chemical compounds, which may form a deposit in or beyond the superficial protoplasm of the corpuscle or may accumulate centrally. These products are therefore either ectoplastic or entoplastic. The chemical capacities of protoplasm thus exhibited are very diverse, ranging from the production of a denser variety of protoplasm, probably as the result of dehydration, such as we see in the nucleus and in the cortical substance of many cells, to the chemical separation and deposition of membranes of pure chitin or of cellulose or of shells of pure calcium carbonate or quasi- crystalline needles of silica. NUCLEUS. The nucleus is probably universally present in the Protozoon cell, although it may have a very simple struc ture and be of very small size in some cases. The presence of a nucleus has recently been demonstrated by means of appropriate staining reagents in some Protozoa (shell- bearing Reticularia or Foraminifera and many Mycetozoa) where it had been supposed to be wanting, but we are not yet justified in concluding absolutely that there are not some few Protozoa in which this central differentiation of the protoplasm does not exist ; it is also a fact that in the young forms of some Protozoa which result from the breaking up of the body of the parent into many small &quot; spores &quot; there is often no nucleus present. In contrast to this it is the fact that the cells which build up the tissues of the Enterozoa are all derived from the division of a nucleated egg-cell and the repeated division of its nucleated products, and are invariably nucleated. The same is true of tissue-forming plants, though there are a few of the lowest plants, such as the Bacteria, the protoplasm of which presents no nucleus. In spite of recent statements (3) it cannot be asserted that the cells or protoplasmic corpuscles of the yeast-plant (Saccharomyces) and of the hyphae of many simple moulds contain a true nucleus. We are here brought to the question &quot; What is a true nucleus 1 &quot; The nucleus which is handed on from the egg-cell of higher plants and Enterozoa to the cells derived from it by fission has lately been shown to possess in a wide variety of instances such very striking characteristics that we may well question whether every more or less distinctly outlined mass or spherule of protoplasm which can be brought into view by colouring or other reagents, within the protoplasmic body of a Protozoon or a Protophyte, is necessarily to be con sidered as quite the same thing as the nucleus of tissue- forming egg-cell-derived cells. Researches, chiefly due to Flemming (4), have shown that the nucleus in very many tissues of higher plants and animals consists of a capsule containing a plasma of &quot; achromatin &quot; not deeply stained by reagents, ramifying in which is a reticulum of &quot; chromatin &quot; consisting of fibres which readily take a deep stain (Fig. I., A). Further it is demonstrated that, when the cell is about to divide into two, definite and very remarkable movements take place in the nucleus, resulting in the disappearance of the capsule and in an arrangement of its fibres first in the form of a wreath (Fig. L, D) and subsequently (by the breaking of the loops formed by the fibres) in the form of a star (E). A further movement within the nucleus leads to an arrangement of the broken loops in two groups (F), the position of the open ends of the broken loops being reversed