Harold Mellor Woodcock

“ The endoplasm is more fluid, and contains numerous inclusions of a granular nature, as well as vacuoles of varying size. In the endoplasm are lodged the nuclei, of which in an adult trophozoite there may be very many; they are all derived by multiplication from the single nucleus with which the young individuals begin life, the number increasing as growth proceeds.
Spore-formation goes on entirely in the endoplasm. The number of spores formed is very variable. ”

“ In those Gregarines (most intestinal forms) which become attached to an epithelial cell, the attachment occurs by means of a minute projection or beak (rostrum) at the anterior end of the sporozoite, which pushes its way into the cell, followed by the first part of the growing germ. This portion of the body increases in size much quicker at first than the rest (the extracellular part) , more or less fills up the host-cell, and forms the well-developed epimerite or secondary attaching organella. ”

“ TRYPANOSOMES, or Haemoflagellates, minute Protozoan parasites, characterized by the possession of one or two flagella and an undulating membrane, and specially adapted for life in the blood of a vertebrate. [1] Of late years considerable progress has taken place in our knowledge of these organisms, research upon them having been stimulated by the realization of their extreme importance in medical parasitology. ”

“ The chief characters which distinguish the Haemosporidia from other Ectospora are the following. They are invariably blood parasites, and for part or all of the trophic period come into intimate relation with the cellular elements in the blood. There is always an alternation of hosts and of generations, an Invertebrate being the definitive host, in which sexual conjugation is undergone and which is to be regarded as the primary one, a Vertebrate being the intermediate or secondary one. The zygote or sporont is at first capable of movement and known as an ookinete. ”

“ The protoplasm in the neighbourhood of each next grows out, as a projecting bud, carrying the nucleus with it. In this manner are formed a number of club-shaped bodies, the merozoites, which are at length set free from the parent-body (IV.) , leaving a certain amount of residual cytoplasm behind. By the rupture of the disorganized host-cell, [2] the fully-formed merozoites are liberated into the intestinal lumen, and seek out fresh epithelial cells. Each is more or less sickle-shaped, and capable of active movements. Once inside a new host-cell, the merozoite grows to a schizont again. ”

“ By the time nuclear multiplication is well advanced or completed, the bodies of the two parent-Gregarines (associates) have usually become very irregular in shape, and produced into numerous lobes and processes. While in some forms (e.g. Monocystis, Urospora, Stylorhynchus) the two individuals remain fairly separate and independent of each other, in others (Lankesteria) they become intertwined and interlocked, often to a remarkable extent (Diplodina) . The sexual nuclei next pass to the surface of the processes and segments, where they take up a position of uniform distribution. ”

“ Male and female gametocytes are distinguished (roughly) by the arrangement of the pigment-grains; in the former, they are fairly evenly scattered throughout the cytoplasm, but in the megagametocytes the pigment tends to be aggregated centrally, around the nucleus. As they become full-grown and mature, however, the gametocytes lose their crescentic form and assume that of an oval, and finally of a sphere. At the same time, they are set free from the remains of the blood corpuscle. ”

“ The seat of the invasion and the mode of parasitism are extremely varied. Practically any organ or tissue may be attacked, excepting, apparently, the testis and cartilage and bone. In one instance at least (that of Nosema bombycis of the silkworm) the parasites penetrate into the ova, so that true hereditary infection occurs, the progeny being born with the disease. The parasites may be either free in some lumen, such as that of the gall bladder or urinary bladder (not of the alimentary canal, or the body-cavity itself) , when they are known as coelozoic forms ”

“ The body is probably not limited by any distinct membrane. The cytoplasm is finely granular and fairly uniform in character. The most interesting point about the morphology is the fact that two chromatic bodies, of very unequal size, are almost invariably to be recognized. The larger nuclear body, which corresponds to the trophonucleus of a Trypanosome, is usually round or oval; the smaller one, representing a kinetonucleus, has the form either of a little rod or of a round grain, and is generally separate from the larger nucleus. ”

“ The parasite invariably destroys its host-cell completely. The latter is at first stimulated to abnormal growth and activity and becomes greatly hypertrophied, the nucleus also undergoing karyolytic changes (fig. 4) . The fatty materials elaborated by the host-cell are rapidly used up by the Coccidian, as nourishment; and at length the weakened and disorganized cell is no longer able to assimilate but dies and is gradually absorbed by the parasite, becoming reduced to a mere enclosing skin or envelope. ”

“ To the brilliant researches of R. Ross and Grassi in the closing years of the 19th century is due the realization of the essential part played by the gnat or mosquito in the life-cycle and transmission of the parasites; and to MacCallum belongs the credit of first observing the true sexual conjugation, in the case of a Halteridium. Since then, thanks to the labours of Argutinsky and Schaudinn, our knowledge of the malarial parasites has steadily increased. Until quite recently, however, very little was known about the Haemosporidia of cold-blooded Vertebrates ”

“ In certain coelomic Gregarines, on the other hand, which remain in very close relation with the host’s tissues, little or nothing of an encystment-process on the part of the parasites is recognizable, the cyst-wall being formed by an enclosing layer of the host (Diplodina) .
The nuclear changes and multiplication which precede sporoblast-formation vary greatly in different Gregarines and can only be outlined here. In the formation of both sets of sexual elements (gametes) there is always a comprehensive nuclear purification or maturation. ”

“ The trophozoite begins life as a small, rounded uninucleate corpuscle, which as it grows, becomes multinucleate. The multinuclear body generally assumes a definite shape, often that of a sausage. Later, the protoplasm becomes segregated around each of the nuclei, giving the parasite a mulberry-like aspect; hence this stage is frequently known as a morula. The uninuclear cellules thus formed are the spores, which are ultimately liberated by the break-up of the parent body. ”

“ We cannot write quite so confidently with regard to the relation of the various pathogenic Trypanosomes to Tsetse-flies (Glossinae) . In the first place experiment has shown that biting-flies, other in all probability than the true, natural hosts, may at times transmit the parasites—as it were—accidentally, if, after feeding on an infected animal, they are allowed to bite a fresh one within a limited time. One very helpful factor in determining which is the principal carrier of any form is the coincidence of the zone of a particular insect with that of any disease. ”

“ 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. ”

“ Nevertheless, in no case does true encystment occur, the sex-cells being developed practically free. The female gametes are large and egg-like; the males, minute and sickle-like, but with no flagellum and apparently non-motile. While many of the zygotes (“amphionts”) resulting from copulation pass out to the exterior, to infect a new host, others, possessing a more delicate investing-membrane, penetrate in between the intestinal cells, producing a further infection (auto-infection) . Numerous sporozoites are formed in each zygote. ”

“ 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 growth and development of an Actinomyxidian have been recently worked out by Caullery and Mesnil in the case of Sphaeractinomyxon stolci. A noteworthy point is the differentiation of an external (covering) cellular layer, which affords, perhaps, the nearest approach to distinct tissue-formation known among Protozoa. This envelope is formed soon after nuclear multiplication of the young trophozoite has begun, and is constituted by two nuclei and a thin, peripheral layer of cytoplasm. It remains binuclear throughout the entire period of development, and serves as a delicate cyst-membrane. ”

“ In the case of free parasites, a well-developed cyst is secreted by the syzygy, which rotates and gradually becomes spherical. A thick, at first gelatinous, outer cyst-membrane (ectocyst) is laid down, and then a thin, but firm internal one (endocyst) . The cyst once formed, further development is quite independent of the host, and, in fact, often proceeds outside it. ”

“ The development of an indifferent ookinete into an indifferent Trypanosome is shown in fig. 7, from which it will be seen that the cytological details are very complex, The indifferent parasites exhibit an alternation of resting, attached phases with active periods, during which they multiply actively and become very abundant in the insect. The male forms, which are very small and the homologies of the micro gametes developed in the blood, appear to die off soon. The female Trypanosomes, on the other hand, grow to a large size, laying up a store of reserve nutriment. ”

“ For, although any one individual only causes the death of a single host-cell, yet the number of the parasites may be so enormously increased by this means, that the entire affected epithelium may be overrun and destroyed. Thus are occasioned grave attacks of coccidiosis, characterized by severe enteritis and diarrhoea, which may end fatally. In the case of the Vertebrates, secondary causes, resulting from the stoppage of the bile ducts, also help to produce death. There is, however, one factor in the endangered animal’s favour. Schizogony cannot go on indefinitely ”

“ The not inconsiderable differences met with among different types are capable of explanation on the ground that certain forms have advanced farther than others along this particular line of evolution. In other words, it is most probable that the Haemosporidia are to be regarded as comprising various parasites which represent different stages intermediate between, on the one side, a Flagellate, and on the other, a typical chlamydospore-forming Ectosporan parasite. ”

“ In each chamber is a pansporoblast, which divides up to produce many spores; hence the spores formed from different pansporoblasts are kept more or less separate. The pansporoblasts originate, in a growing Sarcosporidian, at the two poles of the body, where the peripheral endoplasm with its nuclei is chiefly aggregated. More internally, spore-formation is in progress; and in the centre, pansporoblasts full of ripe spores are found. ”

“ COCCIDIA, an important order of Sporozoa Ectospora, parasites possessing certain very distinctive characters. With one or two possible exceptions, they are invariably intracellular during the entire trophic life of the individual. They always attack tissue-cells, usually of an epithelium, and never blood-corpuscles. ”

“ In other cases, again, the entire sporont-nucleus proceeds to division, and the distinction between somatic and germinal portions does not become manifest until after nuclear multiplication has continued for some little time, when certain of the daughter-nuclei become altered in character, and ultimately degenerate, the remainder giving rise to the sporoblast-nuclei (Diplodina, Stylorhynchus) . Even after the actual sporoblasts (sex-cells) themselves are constituted, their nuclei may yet undergo a final maturation (e.g. Clepsydrina ovata) ”

“ With the bite of an infected mosquito, the minute sickle-like sporozoites are injected into the blood. They rapidly penetrate into the blood corpuscles, in which they appear as small irregular, more or less amoeboid trophozoites. A vacuole next arises in the cytoplasm, which increases greatly in size, and gives rise to the well-known, much discussed ring-form of the parasite, in which it resembles a signet-ring, the nucleus forming a little thickening to one side. ”

“ ENDOSPORA, a natural group or class of the Sporozoa, consisting of the orders Myxosporidia, Actinomyxidia, Sarcosporidia and Haplosporidia, together with various insufficiently-known forms (Sero- and Exosporidia) , regarded at present as Sporozoa incertae sedis. ”

“ Agglomeration consists in the grouping or union together of several Trypanosomes around a common centre; this leads to the formation of rosette-like clusters, or even of large masses composed of several rosettes. The end by which the parasites join is typically, in the case of Trypanosoma, the non-flagellate (anterior) end. If a favourable change in the surrounding medium sets in, the Trypanosomes are able to undergo the reverse process, namely dis agglomeration; the parasites liberate themselves and the rosette is dissolved. ”

“ In the vast majority of Gregarines, however, the life-cycle is limited to gametogony and sporogony. A very general, if not indeed universal, prelude to gametogony is the characteristic and important feature of the order, known as association, the biological significance of which has only lately been fully brought out (see H. M. Woodcock [31] ) . In normal association, two individuals which are to be regarded as of opposite sex, come into close contact with each other and remain thus attached. The manner in which the parasites join varies in different forms ”

“ 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. ”

“ It is very interesting to note that, in the two last cases, unless this association of the microgametocyte with the megagametocyte occurs, neither can the former produce male elements (microgametes) nor can the female individual maturate and become ready for fertilization. (Concerning this question of association see also Gregarines.)
In sporogony, great variation is seen with respect to the number of spores and sporozoites formed; and, as in Gregarines, these characters are largely used for purposes of classification, under which heading they are better considered. ”

“ The small Trypanosomes resulting from either of these modes of division differ from typical adults by their stumpy, pyriform shape, the position of the kinetonucleus near the flagellar end of the body, and the absence, during the first part of their youth, of an undulating-membrane. At this period they have, in fact, what may be termed a “pseudo-Herpetomonadine” aspect. These young individuals can themselves multiply by equal binary fission, giving rise to little fusiform parasites; with growth, these gradually assume the adult appearance. ”

“ Most forms attack the intestinal wall, often destroying its epithelium over considerable areas; but one genus, Sphaeractinomyxon, inhabits the body-cavity of its host. The researches of Caullery and Mesnil (10-12) and of Léger (28 and 29) have shown that the parasites exhibit the typical features of the Endospora, and the spores possess the characteristic polar-capsules of the Myxosporidian spore, but differ therefrom by their more complicated structure. ”

“ Around this some of the cytoplasm condenses, the rest forming a residuum. The sporoblast or gamete thus formed is completely isogamous and normally conjugates with the like one from the other associate, when a single zygote results which becomes a spore containing eight sporozoites, in the ordinary manner. Sometimes, however, the septum between the two halves of the cyst does not break down, in which case parthenogenesis occurs, each sporoblast developing by itself into a small spore. ”

“ As in the case of Trypanosomes (q.v.) , normally—that is to say, when in an accustomed, tolerant host, and under natural conditions—Haemosporidia are non-pathogenic and do not give rise to any ill-effects in the animals harbouring them. When, however, the parasites gain an entry into the blood of man or other unadapted animals, [3] they produce, as is well known, harmful and often very serious effects. There are three recognized types of malarial fever, each caused by a distinct form and characterized by the mode of manifestation. ”

“ The effects of a Coccidian infection upon the host as a whole depend largely upon the extent to which endogenous multiplication of the parasites takes place. On the one hand, schizogony may be so limited in extent as not to cause appreciable injury to the host. This seems to be often the case in forms infecting Molluscs and Arthropods. On the other hand, where schizogony is rapid and prolonged, the results are often serious. ”

“ Trypanosomes vary greatly with regard to size; even in one and the same species this variation is often noticeable, especially under Mon, , w, oD, different conditions of life. The common Trypanosoma rotatorium of frogs (fig., A and B) is, taking it all in all, one of the largest forms so far described. Its length (inclusive of the flagellum) varies from 40-60 p, while its greatest width (including the undulating-membrane) is from 8-30, a; in the very wide individuals breadth is gained more or less at the expense of length. ”

“ The two conjugating elements unite completely, cytoplasm with cytoplasm and nucleus with nucleus, to form the definitive sporoblast or zygote. The protoplasm assumes a definite outline, generally that of an ovoid or barrel, and secretes a delicate membrane, the ectospore. This subsequently becomes thickened, and often produced into rims, spines or processes, giving rise to the characteristic appearance of the Gregarine spore. Internal to the ectocyst, another, thinner membrane, the endocyst, is also laid down. ”

“ In the stomach of an Anopheles, the gametocytes quickly proceed to gamete-formation. The nucleus of the microgametocyte divides up, and the daughter-nuclei pass to the periphery. The surface of the body grows out into long, whip-like processes, of which there are usually 6 to 8 (probably the typical number is 8) ; each is very motile, in this respect strongly resembling a flagellum. This phase may also develop in drawn blood, which has, of course, become suddenly cooled by the exposure ”