Merozoite

“ When full grown the nucleus divides by repeated fission till a variable number, generally about thirty, are produced. The schizont now divides into as many merozoites as there are nuclei, and is then full grown. The cells burst, the merozoites are set free, and, entering other cells, may develop in two ways, either into schizonts again or into sporonts. The sexes of the sporonts can be distinguished; in the male the protoplasm is clear, but in the female it is crowded with reserve food material. ”

“ When the schizogony ceases, the merozoites, that had penetrated the epithelial cells and become trophozoites there, consist of two kinds of differently constituted individuals. One kind possesses a clear cytoplasm (fig. 67, XII) , the other an opaque, richly granular cytoplasm (fig. 67, XI) , while both possess a vesicular nucleus with a karyosome. In order to continue their development, the more granular individuals must be fertilized, and are therefore termed either female gametes or, on account of their size, macrogametes. ”

“ The micro-gametes soon liberate themselves from the residual cytoplasm of the parent and swim away in search of a megagamete; each is a very slender, wavy filament, composed largely of chromatic substance. The finer details of structure of the microgamete of a malarial parasite cannot be said, however, to be thoroughly known, and it is by no means impossible that its structure is really trypaniform, as, according to Schaudinn’s great work, is the case with the merozoites and sporozoites. ”

“ The principal stages of the avian plasmodium closely resemble those of the malarial parasites of man. In its earliest stage P. relictum is unpigmented, but soon the trophozoite grows and becomes pigmented, meanwhile displacing the nucleus of the avian red-blood corpuscle, a characteristic feature, distinguishing it from Halteridium. Schizonts are formed, each of which gives rise to about nine merozoites in the circulating blood. Sexual forms or gametocytes also occur in the blood. These develop in Culex fatigans, C. pipiens and C. nemorosus. ”

“ The young trophozoite is characterized by its signet-ring appearance, there being a large space in the centre of the body. As the parasite grows this space disappears, and the pigment is deposited in its protoplasm. When full grown, multiplication by schizogony takes place. The nucleus, hitherto single, multiplies by repeated division, each daughter nucleus becoming the centre of a merozoite. A blood corpuscle containing a number of such merozoites represents the characteristic rosette of the malaria parasite. The host cell then ”

“ The liberated merozoites proceed to infect fresh blood corpuscles and a new endogenous cycle is started.
After asexual multiplication has gone on for some time, sexual forms become developed. According to Schaudinn, the stimulus which determines the production of gametocytes instead of schizonts is the reaction of the host (at the height of a fever period) upon the parasites. A young trophozoite which is becoming a gametocyte is distinguished from one which gives rise to a schizont by its much slower rate of growth, and the absence of any vacuoles in its cytoplasm. ”

“ Usually, the body divides up, by a process of multiple fission (fig. 10) , into a few (up to eight) daughter-individuals; but in a new genus (Eleutheroschizon) , Brasil (3) finds that a great number of little merozoites are formed, and a large amount of vacuolated cytoplasm is left over unused. ”

“ The protozoa parasitic in blood of the vertebrate host undergo a vegetative or non-sexual multiplication, the process being then known as schizogony; but in order to ensure their further existence, and their passage from one vertebrate host (intermediary host) * [1] to another, a further cycle in a cold-blooded invertebrate host (definitive host) is necessary, during which the sexual cycle or sporogony takes place.
In the vertebrate blood the young parasite is known as a schizont. The products of the asexual fission are known as merozoites ”

“ Gregarines in which the sporozoites are naked) constitute in reality nothing more or less than a schizogonous generation of these Cephalopodan parasites, which have thus an alternation of true hosts. The ripe sporocysts from the Cephalopod are eaten by a particular crab (e.g. Portunus or Inachus, according to the parasite) , the sporozoites are liberated and traverse the mucous membrane of the intestine, coming to rest in the surrounding lymphatic layer. Here a large “cyst” is formed, projecting into the body-cavity, the contents of which give rise to a great number of merozoites. ”

“ Eight to fourteen merozoites are usually formed by each schizont, twenty have been found, while in cases of intense infection when space has become limited, the number may be only four.
After a number of generations of merozoites have been formed, a limit is reached both to the multiplicative capacity of the parasite and to the power of the bird to provide the invader with food. Consequently, resistant forms of the parasite are necessary, and the trophozoites begin to show sexual differentiation instead of forming schizonts, that is, gametogony commences. ”

“ Besides the difference in the time taken by the complete process of schizogony in the various species (see above) , there are distinctions in the composition of the rosettes. Thus, in Laverania, the number of merozoites formed is very variable; in Plasmodium vivax (the tertian parasite) there are only few (9 to 12) merozoites, but in P. malariae (the quartan form) they are more numerous, from 12 to 24. ”

“ Closely allied to the Coccidiidae are the Haemosporidae, dwellers in the blood of various cold-blooded Vertebrates, [109] and entering the corpuscles as sporozoites or merozoites to attain the full size, when they divide by schizogony; they are freed like those of the next family by the breaking up of the corpuscle. The merozoites were described by Gaule (1879) as "vermicles" ("Würmchen") , and regarded by him as peculiar segregation-products of the blood; though Lankester had described the same species in the Frog's blood as early as 1871, with a full recognition of its true character. ”

“ The male elements (microgametes) are formed in a manner essentially comparable to that in which the formation of merozoites takes place. Although the details of the nuclear changes and divisions vary somewhat, the end-result is similar, a number of little nuclear agglomerations being evenly distributed at the surface (VII. ♂) Each of these elongates considerably, becoming comma-shaped and projecting from the gametocyte. Nearly all the body of the male gamete (VIII. ♂) consists of chromatin, the cytoplasm only forming a very delicate zone or envelope around the nucleus. ”

“ This parasite lives in the intestinal villi of dogs, cats, and the polecat (Mustela putorius, L.) . According to Stiles,188 the oöcyst divides into two equal ellipsoidal portions or sporoblasts which become spores and then each forms four sporozoites. The oöcysts of this species vary from 22 μ to 40 μ in length and from 19 μ to 28 μ in breadth. Each spore is 10 μ to 18 μ long and contains four sporozoites. The parasites live and multiply, not only in the gut epithelium, but also in the connective tissue of the intestinal submucosa. Wasielewski has seen merozoites in the gut of the cat. ”

“ These zoospores are amoeboid; they soon secrete a shell and reveal themselves as megalospheres, the original state of the megalospheric forms. In the adult megalosphere the solitary nucleus disappears and is replaced by hosts of minute vesicular nuclei, formed by the concentration of chromidia. Each nucleus aggregates around it a proper zone of dense protoplasm; by two successive mitotic divisions each mass becomes quadri-nucleate, and splits up into four biflagellate, uninucleate zoospores. These are pairing-cells or gametes, though they will not pair with members of the same brood. ”

“ A dredger who should have no other information as to the existing population of the world, except what he could gather from the deposits formed under several hundred fathoms of water, would necessarily have very inadequate conceptions of the matter. In like manner a geologist who should have no other information as to the animal life of the Mesozoic ages than that furnished by some of the thick beds of white chalk, might imagine that he had reached a period when the simplest kinds of protozoa predominated over all other forms of life ”

“ Many species are "epiphytic" or "epizoic," dwelling on plants or animals, and sometimes choice enough in their preference of a single genus or species as host. Others again are "moss-dwellers," living among the root-hairs of mosses and the like, or even "terrestrial" and inhabiting damp earth. The Sporozoa are internal parasites of animals, and so are many Flagellates, while many Proteomyxa are parasitic in plant-cells. The Foraminifera (with the exception of most Allogromidiaceae) are marine, and so are the Radiolaria; while most Heliozoa inhabit fresh water. ”

“ In both cases it is the forms with a single nucleus, corresponding to megalospheric forms that so pair, and the brood-formation is, mutatis mutandis, the same as in these forms. Similar individuals may reproduce in the same way, in both genera, without this plastogamic pairing, which is therefore, though probably advantageous, not essential. If pseudopodiospores form their shells while near one another, they may coalesce to form monsters, as often happens in Orbitolites. [81]
The direct economic uses of the Foraminifera are perhaps greater than those of any other group of Protozoa. ”

“ The lowest group, being destitute of organic remains, is called azoic, (from α privitive and ζωὁς,) that is, wanting in the traces of life; and corresponds to the crystalline group on the other side of the section, embracing gneiss, mica slate, limestone, and clay slate, of unknown thickness. The protozoic group corresponds to the palæozoic of the right hand side, and embraces lower and upper Silurian, Devonian, or old red sandstone, the carboniferous group, and the Permian, or lower new red; the whole in Great Britain not less than thirty-three thousand feet thick. ”

“ The great number of organisms, such as the Foraminifera which secrete calcareous tests or "shells" causes the slime on the continental 6 shelves to be calcareous and in the condition to form limestone if cemented or subjected to sufficient pressure. In the deep sea, where the hard parts of dead organisms fall through many thousands of feet of water, their more soluble portions are removed and the bottom is covered throughout vast areas with a pinkish clay composed of the more insoluble residue of the calcareous shells and the cases of silica-secreting animals and plants. ”

“ Though the abundance and wide distribution of Eozoon, and the important part it seems to have acted in the accumulation of limestone, indicate that it was one of the most prevalent forms of animal existence in the seas of the Laurentian period, the non-existence of other organic beings is not implied. On the contrary, independently of the indications afforded by the limestones themselves, it is evident that in order to the existence and growth of these large Rhizopods, the waters must have swarmed with more minute animal or vegetable organisms on which they could subsist. ”

“ Beyond the upper part of the outer slopes of the continental shelves, the pelagic deposits are largely made up of shells and skeletons of marine organisms which live in the surface-waters. Pelagic molluscs, foraminifera, and algæ secrete shells of lime carbonate, while diatoms and radiolarians secrete shells of silica. When the organisms die, they sink to the bottom with their shells, and these mineral matters of organic origin are mingled with the volcanic products which are universal over the sea-floor. ”

“ Beyond the reach of waste from the land the bottom of the deep sea is carpeted for the most part with either chalky ooze or a fine red clay. The surface waters of the warm seas swarm with minute and lowly animals belonging to the order of the Foraminifera, which secrete shells of carbonate of lime. At death these tiny white shells fall through the sea water like snowflakes in the air, and, slowly dissolving, seem to melt quite away before they can reach depths greater than about three miles. Near shore they reach bottom, but are masked by the rapid deposit of waste derived from the land. ”

“ The use of the microscope is essential in the case of some of the harder tuffs, where there has been a little alteration. Here casts of foraminifera may be observed, although no carbonate of lime is indicated by an acid. In some localities where no organic remains are evident in the tuff, fine waterworn gravel is to be noticed.
These deposits are composed as a rule of sub-angular fragments of semi-vitreous basic or basaltic rocks and of palagonite, together with fragments of plagioclase and pyroxene, the interspaces being filled with fine debris of the same materials. ”

“ Sporozoa in the persistence of the encysted stage are very plant-like, though they are often intracellular and are parasitic in living Animals. On the other hand, the Infusoria for the most part answer to all the physiological characters of the Animal world, but are single cells, and by the very perfection of their structure, all due to plasmic not to cellular differentiation, show that they lie quite off the possible track of the origin of Metazoa from Protozoa. ”

“ So long as the imbedding mass continues soft and incoherent, shells, corals, etc., can be recovered in a condition similar to that of recent specimens, except that they may have become bleached in colour and brittle in texture, owing to the removal of organic matter intimately associated with the lime, and that their cavities may have been filled with sand or silt washed into them, or with calcite or calcareous spar introduced in solution in water. But if the containing mass has become a hard stone, the material filling the interior of our shell or coral has experienced a similar change ”