elongated and pointed at one extremity, and also exhibits motility. It is called the travelling vermicule or zygote. It works its way through the mass of blood and the stomach wall, finally arriving at the outer surface of the organ. Here it becomes encysted and develops a large number of delicate spindle-shaped cells called blasts. The capsule then ruptures, and the blasts are discharged into the body cavity of the insect. These are finally carried by the circulation to the salivary gland of their host, whence they pass through the salivary duct into the proboscis of the insect. When the mosquito again bites an individual the salivary secretion passes into the wound, and so the blasts enter into the circulating blood of the human host and give rise to the amoeboid organisms. These enter the red corpuscles and pursue the developmental cycle described above.

The mosquito, or gnat, belongs to the genus culicidæ, of which numerous species are extant. From a medical point of view, only two varieties are important, the culex and the anopheles. In the early stages of existence, viz., as ovum, larva, and pupa, the mosquito is aquatic; but the mature insect or imago is aerial.

The culex or the common house mosquito is found almost everywhere. Its ova are usually arranged in boat-shaped masses, and the larvæ usually live in pots, tubs, and other artificial collections of water. In the adult insect the wings are unspotted, the palpi short, and, when at rest, it is usually found parallel to the wall.

The anopheles, on the other hand, is rarer, and serves as the definitive host of the malarial parasite. It is not domestic in habits like culex, but breeds in puddles, ricefields, or other similar collections of water. The ova are generally in star-shaped or irregular masses, but the

grouping is never boat-shaped. The full-grown anopheles is more graceful than culex, its palpi are long and the wings are dappled with dark spots. It is nocturnal in its habits, and only visits habitations in the evening. Both sexes feed on fruit; but the female alone sucks blood from men and other animals.

Varieties of the Malarial Parasite. - Three distinct species of the malarial parasite are recognised, each of which gives rise to a specific type of infection.

1. The Parasite of Tertian Fever.-The tertian parasite is fully matured in about forty-eight hours, and the sporulation of a group of organisms occurs at the same time, giving rise to the characteristic paroxysms every third day. During its development certain forms are evolved, which are highly characteristic.

If a specimen of blood from a case of tertian fever be examined at or before the period of rigor, it will be observed that many of the red corpuscles contain small hyaline forms actively changing their shape and position

in the interior of the cells. These bodies represent the earliest stages of the parasite, and are derived from the segmentation of the preceding group of the organisms. As the parasite increases in size, pseudopodia are protruded and retracted, so that various characteristic forms arise. Usually a single parasite is contained in a cell, although occasionally two or more are seen.

The next stage in the development of the organism is the collection of dark-brown pigment, which it derives from the hæmoglobin of its host. The pigment granules are more marked at the periphery than at the centre, and their active "brownian" movement constitutes an important feature of the organism.

As the parasite matures, it becomes larger in size, more pigmented and less amoeboid, until the latter characteristic is altogether lost. The pigment now becomes coarser, and, instead of being distributed eccentrically, is now scattered through the protoplasm of the parasite. the same time the infected blood cells are decolorised, so that they appear as pale rims encircling the mature parasite.

At

The nutritive supply within the blood corpuscle being exhausted, the parasite now proceeds to sporulate. The pigment collects into a central mass, and fine striations are seen to extend from the periphery to the centre, so that the parasite assumes the form of a "rosette". This is the appearance which is observed on examining the blood just before the next paroxysm. The protoplasm of the parasite finally breaks up into twelve to thirty segments, each of which contains a portion of nuclear matter, and is called a spore. The envelope of the red corpuscle ultimately ruptures, and the spores are set free in the blood plasma, in which situation they are seen forming an irregular group round the pigmented mass.

Sooner or later they re-enter red corpuscles, and initiate a similar cycle of forty-eight hours' duration. Most of the liberated pigment is taken up by the phagocytes or deposited in various organs.

Besides these forms, which constitute the asexual cycle of the parasite, certain large pigmented spheres are seen, especially in the apyrexial period. They are the gametocytes or gametes, and are derived from the parasite losing its corpuscular envelope prior to sporulation, which, however, does not take place. Some of these gametes emit three or four long delicate flagella, which actively lash about in the fluid, and finally break away from the parent cell and are lost from view.

Certain other spheres may also be seen which are larger and do not flagellate. Their pigment is less active and usually arranged in the form of a ring. They are the female elements, whereas the flagella of the flagellating body represent the male sexual organs, which bring about the fertilisation of the former in the manner already described.

2. The Parasite of Quartan Fever completes its cycle of development in seventy-two hours, and the fever is therefore more benign than the tertian. The developmental changes in the parasite are essentially similar to those described above, but a few morphological differences may be observed. Thus, the parasites are smaller, their movements less active, and the pigment granules coarser. The sporocyte or rosette body is made up of from six to twelve spores, and the red corpuscles do not become pale.

3. The Parasite of Malignant or Estivo-Autumnal Fever. In this type of fever the marked periodicity of the above two varieties is wanting, as larger numbers of this organism exist in the blood in different stages of development. Segmentation thus takes place at different

intervals and gives rise to that irregularity which is so characteristic of this disease.

The developmental cycle of this parasite, moreover, is difficult to follow, as the latter portion of its life-history is carried on in the internal organs. However, certain wellmarked differences can be made out. The young parasites are very small but very active, and the ring forms are common. The red corpuscle usually contains two or more parasites and assumes a characteristic "brassy" tint.

An important feature is the presence of "crescentic bodies". They appear in the peripheral circulation after the fever has lasted for some time, and are peculiarly resistant to the influence of quinine. The crescents are nothing else than the gametes, male and female. When examined under the microscope the crescents change into spherical bodies, some of which proceed to flagellate and the others do not. The manner of fecundation is essentially the same as in the tertian and quartan types of fever.

Relations to Disease.-The hamamaba malaria has not been cultivated outside the body, but its causal relation to the disease may be taken as fully established. Thus, the parasite is always present in the blood of malarial patients and is met with in no other condition. Its cycle of development corresponds with the clinical course of the disease, and the destruction of the red corpuscles fully explains the subsequent anæmia. Quinine, which is a well-known amoebic poison, kills the parasite and cures the malarial fevers. When a healthy person is inoculated with a minute quantity of malarial blood, not only the infection but also the specific type of the disease is reproduced. A malarial person can mix freely with healthy persons without infecting them with the disease, provided there are no anopheles about to convey the parasite.