The history of the ticks (Rhipicephalus appendiculatus) employed for transmission of East Coast fever is detailed in Part I. During the 1st day the ticks generally become attached and are partly held in place by a kind of cement substance (Fig. 1).

Penetration usually occurs at some time during the 2nd day, and the cellular elements of the corium quickly respond. There is a local swelling of the capillary endothelium, an infiltration with polymorphonuclear leucocytes and a slight increase in eosinophiles. More cement substance is produced which hardens about the edges. Some of it enters through the wound and spreads out in the corium along lines of least resistance (Fig. 2).

Later on the eosinophiles increase greatly in number and in turn give place to macrophages which ingest some of the cement substance. Mast cells also become more numerous. The area of reaction extends 2 or 3 mm. from the tip of the proboscis. Marked cytolysis of fibroblasts, of histiocytes and of the swollen endothelial cells occurs. The latter leads to haemorrhage (Figs. 6, 8, 9). The red blood cells and leucocytes are, however, not destroyed. They are sucked up by the tick, the leucocytes first because they accumulate before the haemorrhages take place.

Twenty-five of the sixty-six ticks examined were found to contain parasites in their salivary glands, but no parasites were seen in the sixty-six pieces of skin excised while the ticks were attached. Soon after the parasites enter the blood stream and spread throughout the system the corium of the skin is invaded by them like other parts of the body (Table III).

The medium-sized lymphocytes are the principal cells parasitised; but parasites are also found in lymphoblasts, large and small lymphocytes and occasionally in parenchymatous liver cells. We never saw them in vascular endotheial cells. For this reason we are unable to substantiate the view, which has apparently never been seriously questioned, that they undergo schizogony in this situation. The first agamonts were noted in lymphocytes in gland smears the day the fever first exceeded 103° F. The gamonts appeared about a day later and the intra-erythrocytic forms soon after. Taking the average of a number of cases: (1) The temperature first surpassed 103° F. on the 9·8th day after infestation with ticks. (2) The first Koch bodies within lymphocytes were observed on the 14·8th day, that is to say, 5 days after the fever commenced. (3) The first parasites within the red cells were noted 1·6 days later, on the 16·4th day after infestation.

The cycle which the parasite undergoes in changing from the agamonts to the intra-erythrocytic forms is illustrated in Diagram II (fully described on p. 36).

The alterations in the tissues caused directly or indirectly by the parasitic invasion were studied principally in specimens secured at autopsy from animals killed in extremis. The changes which apparently dominate relate to the lymphatic system. The medium-sized lymphocytes and to a less extent the small and large lymphocytes and lymphoblasts, first in the lymph glands and later in all parts of the body, become infested with Koch bodies which grow in their interior apparently without causing any severe injury. The distribu tion of Koch bodies is recorded in Table III, and their incidence in the lymph glands and in the liver in Tables IV and V. The evidence presented does not support the current conception that the parasites serve as a powerful stimulus to the multiplication of lymphocytes. The enlargement of lymph glands is caused more by oedema than by increase in their number. But there is a marked redistribution of lymphocytes. Their number is reduced in the lymph glands and in the peripheral blood stream and increased in the blood vessels of the liver and some other organs. Perivascular lymphocytic infiltrations are also a notable feature of the reaction in the liver, kidneys, adrenals and several other organs, but they are of rare occurrence in the nervous system. It is uncertain how far they are formed by the emigration of lymphocytes from the blood stream and by the proliferation of lymphocytes previously present perivascularly.

Small haemorrhages constitute a second alteration of importance. They are particularly prone to occur in the epicardium and endocardium, but may be found in almost any part of the body. Evidence is lacking that these haemorrhages play a part in the genesis of the perivasoular lymphocytic infiltrations. The latter are far more widespread and are chiefly observed in locations where haemorrhages are seldom seen.

Oedema is a third type of reaction. It is constantly found in the lymph glands and occasionally in the lungs, especially in the later stages of the disease.

Generally speaking, therefore, the lesions reported in this paper are acute in type. The functional activity of the several organs (except the lymph glands) has not been impaired sufficiently severely or over a time long enough to leave distinct structural changes in the parenchymatous organs; but the specimens examined were not collected from protracted cases of East Coast fever or from animals under suspicion of having undergone repeated attacks of the disease.

It is the parasites in the red blood cells which are capable of continuing the cycle if they are ingested by ticks able to act as vectors. We have recog nised three principal groups: (1) small spherical-ovoid parasites which are the first to make their appearance, (2) tailed parasites, and (3) plunip forms, both of which develop later and become more numerous than the spherical-ovoid parasites. Possible stages of transition are represented in Diagram I (p. 37). Though many parasites undergo differentiation and some probably divide, a variable number degenerate within the erythrocytes.

We question the validity of the genera into which the family Theileriidae are customarily divided on account of the inadequacy of the data on which they are based.