November 21, 1913] 



SCIENCE 



729 



deduce from the one the etiology of the 

 other, which was confirmed by Grassi and 

 others. The Plasmodium prcecox is, in 

 many stages, so like human malaria that it 

 can only be differentiated by the presence 

 of the oval nucleus of the bird's red cor- 

 puscles. The life-cycle is very complex, 

 part taking place in the blood of the bird, 

 and another part (sexual reproduction) in 

 the body of a mosquito. This parasite was 

 first seen by Grassi in 1890; it is very 

 widely distributed, and is very deadly to 

 birds. 



Human malaria has been known for cen- 

 turies. Varro, who knew a good deal 

 about what we should now call hygiene, 

 more than a century B.C., thought that ma- 

 larial fevers were due to invisible animals, 

 which entered the body with the air in 

 breathing, and Vitruvius, Columellus and 

 Paladins were of the same opinion. Now 

 we know that the mosquito is again the car- 

 rier, and that the sexual part of the para- 

 site's cycle takes place in it, but whether 

 the mosquito alone can account for all the 

 phenomena of malaria is not yet quite cer- 

 tain. 



There are three varieties of malaria in 

 man — the tertian, quartan, and quotidian; 

 in the tertian the cycle of the parasite in 

 the body takes forty-eight hours, and in 

 quartan seventy-two hours, and in perni- 

 cious malaria the fever is very irregular, 

 but continuous. Whether there are three 

 different parasites, or only one, which is 

 altered according to its environment of 

 host, climate, etc., is still apparently uncer- 

 tain. Laveran and Metchnikoff believe in 

 the specific unity of the parasite, whereas 

 some observers want as many as five differ- 

 ent species. 



Just as in human malaria the pernicious 

 form is distinguished by the elongated form 

 of its gametes, so in birds there is a para- 

 site which is distinguished, in the same 



way, from Plasmodium prcecox by its very 

 elongated gametes. This parasite is called 

 Hcemoproteus Danilewski. Its development 

 is unknown; it begins as a tiny irregular 

 body in the red corpuscles of the bird, then 

 it grows in the long axis of the cell and 

 turns round the end of the nucleus. It is 

 possible in these parasites to follow the 

 process of impregnation, which normally 

 takes place in some insect. By taking the 

 blood when full of the long, fully-grown 

 gametocytes, and keeping it for a time out- 

 side the body, this process can be followed. 



First of all, the gametocytes escape from 

 the blood-corpuscles and roll themselves up 

 into a ball. Some of thase remain quiet — 

 the females, curiously, the macrogameto- 

 cytes — whilst in the microgametocytes ac- 

 tive movements are seen ; then tailed proc- 

 esses are seen projecting from its surface, 

 which at last get free and wander about in 

 the blood, this constituting the origin of 

 the microgametes from the microgameto- 

 cyte. They then find a macrogamete, and 

 penetrate into it and fertilize it. This fer- 

 tilized macrogamete then alters its shape 

 and becomes an ookinete, with the remains 

 attached containing the pigment. It may 

 enter a red corpuscle, but it usually breaks 

 up, because it finds it is not in the stomach, 

 of the insect it intended to be in, but be- 

 tween two pieces of glass. 



From Hcemoproteus it is easy to pass to 

 a rare and undetermined parasite of the 

 blood of birds called a Leucocytozoon. It 

 occurs in the blood in the form of a long, 

 spindle-shaped, unpigmented body. Very 

 little is known of it except that it is found 

 in its sexual forms. The earliest observers 

 of this parasite — Danilewsky and Ziemann 

 — believed the host-cell to be a leucocyte 

 (hence the name), but Laveran has shown 

 that it is a red corpuscle. 



We now come to a group of parasites of 

 great practical importance, the Babesias, 



