THE SPOROZOA 253 



denser protoplasm, with most of the pigment, and with the pale nucleus ; 

 the other with lighter protoplasm and less pigment, containing the dark 

 nucleus, which now proceeds to divide as in schizogony and gives rise to 

 a number of merozoites. The latter are the starting-point of fresh 

 schizogonous cycles of generation, bringing about a return of the fever. 

 The denser portion of the gametocyte with the pale nucleus is abandoned 

 as residual protoplasm and breaks up. Only the female gametocytes are 

 capable of reproducing themselves in this way. The microgametocytes, 

 with their greatly enlarged nucleus and reduced bulk of protoplasm (see 

 p. 248), are believed by Schaudinn to die off if they do not undergo their 

 natural course of development in the intermediate host. 



Attention must also be drawn to another point which is not yet 

 fully explained. In mosquitos infected by these parasites, in addition to 

 the ordinary cysts containing sporozoites, there occur also other cysts of 

 about the same size, but very different in appearance, as they are filled 

 with masses of dark brown pigment, which is quite different in appearance 

 from the melanin-pigment of the parasites. Ross was the first to describe 

 these bodies in Culex infected by Haemoproteus danilewskyi in birds, but 

 they occur also in Anopheles infected with human malarial parasites. 

 They have received various designations: "yellowish -brown bodies," 

 Grassi ; " black spores," Ross ; " brown spores," Nuttall. Ross regarded 

 them as resistent cysts, destined to develop in some unknown way, 

 and Grassi at first thought they were intended to spread the infection 

 amongst successive generations of mosquitos. It is, however, sufficiently 

 well established that mosquitos neither come into the world infected with 

 these parasites, nor acquire them in any other way but from the blood of 

 their prey. Most authorities incline now to the later opinion of Grassi, 

 and regard the yellowish-brown bodies as degenerate cysts of the ordinary 

 kind, the pigment being produced by a protoplasmic mass consisting partly 

 of the residual substance, partly of abortive sporozoites left behind in the 

 cyst. This conclusion receives indirect support from the observations of 

 Schaudinn [5 la] upon the degenerated oocysts of Cyclospora (see p. 273). 



In the above description of the life-cycle, the mosquito has been referred 

 to as the " intermediate host." Many authorities, however, such as Grassi, 

 Mesnil, Laveran, and others of great note, consider that the Invertebrate 

 host, the mosquito, should be regarded as the "principal" or "definitive " 

 host, and the Vertebrate, man, as intermediate, chiefly on the ground that 

 the sexual phases of the parasite are passed through in the former. In 

 considering these conceptions, it should be made clear at the outset in what 

 sense the term " principal host " is used. If it be employed in the sense 

 of the primary or primitive host, then it must certainly be applied to the 

 Vertebrate, for, while all the Haemosporidia have a Vertebrate host, there is 

 at present no evidence whatever, in the case of many of them, that an 

 Invertebrate host has been acquired as a means of dispersal (see below, 

 p. 263). The Haemosporidia as a whole must be considered as parasites 

 of Vertebrates in the first instance, which have in some cases adapted 

 themselves for certain phases of their life-history to a secondary Invertebrate 

 host. If, on the other hand, the term "principal host" be employed in a 

 physiological or functional sense, it is again the Vertebrate that must be 



