become sporonts; the macromerozoites may again repeat the cycle and become once more macromerozoites or micromerozoites.
These sporonts become differentiated into male and female gametocytes and appear in the circulating blood, but only develop further in the invertebrate host. In the intestine of the invertebrate the male and female gametes, having escaped from their cells as motile vermicules, become approximated and fuse to form a zygote (but in the case of H. stepanowi of the tortoise four microgametes are first produced by the male gamete). The zygote then apparently becomes an oökinete, encysts, and grows into a large oöcyst containing a variable number of sporocysts, each of which contains six or eight sporozoites, which again in some manner pass into the intermediary host. In the case of the leech the exact manner in which this is performed is not known, but in the case of H. muris, which develops in the rat-mite (Lelaps echidninus), the mite is devoured by the rat, and the sporozoites, liberated in the rat's intestine, pass through the wall of the gut into the blood-stream, re-enter the liver cells, and become schizonts.
Fig. 240.—H. canis; endocellular in leucocytes and free sporonts.
(After Wenyon.)
In the leucpcytic gregarines, of which the beat known is H. canis(Fig. 246), described by Christophers and Wenyon, schizogony takes place in the bone marrow and spleen of the dog, and is, as described above, of two distinct types. The macromerozoites in this case pass into the blood, are taken up by the leucocytes, and become gametocytes. Sporogony takes place in the body tissues of the tick Rhipicephalus sanguineus. The dog probably becomes infected by eating the tick.
Suborder Adeleidæ.—This suborder is of little interest from the standpoint of human pathology. It consists of intracellular parasites of invertebrates which differ in their life-history from the foregoing in that the sporonts do not remain separate as in Coccidium schubergi, but associate in pairs, the male sporont becoming attached to the female.
Order iii.—Hæmosporidia
These parasites are parasitic during the greater part of their life cycle; but they exhibit, as do the hæmogregarines, an alternation of generation, sporogony generally taking place in the digestive tract of some invertebrate.
The order comprises the genera Plasmodium, Hœmoproteus, Leucocytozoon, and Piroplasma.
1. In the genus Plasmodium the young schizonts occur within red blood-cells, exhibit amœboid movement, and pro-
