THE SPOROZOA 261 



tissues in which it comes to rest and becomes encysted as an oocyst. 

 In all cases that have been recently studied, the oocyst is formed 

 in the epithelium of the digestive tract, either of the same or of an 

 intermediate host. 



With regard to the sporogony, two types can be recognised, 

 the differences between which depend upon whether the oocyst is 

 actively parasitic upon the tissues in which it encysts, as in the 

 malarial parasites, or whether it forms round itself a tough 

 protecting membrane within which it is more or less independent 

 of its host or of external conditions, as in Lankesterella (Fig. 75, 6). 

 The latter case is undoubtedly the more primitive, and does not 

 differ essentially from the state of things seen in the Coccidia. In 

 the oocyst of Lankesterella the number of sporoblasts is relatively 

 small, and each sporoblast appears to give rise to a single sporozoite 

 only. This condition is related, in this instance at least, with 

 absence of an intermediate host. Sporogony and schizogony here 

 go on in the same animal. On the other hand, in the malarial 

 parasites of birds and man, perhaps of all warm-blooded animals, 

 sporogony takes place, as in Laverania, in an intermediate host, upon 

 which the oocyst is actively parasitic. The enveloping membrane 

 in these forms is very thin according to Grassi it is formed by 

 the host and not by the parasite and the zygote grows greatly 

 in size, forms a number of sporoblasts, and each sporoblast gives 

 rise to very numerous sporozoites, as described above for Laverania. 

 This great increase of reproductive power must be regarded as a 

 secondary adaptation of a kind common in all forms of parasitic 

 organisms, whereby the chances of disseminating the parasite 

 amongst fresh hosts are much heightened by the vast number of 

 germs produced from each- individual. 



In no case, however, are sporocysts secreted within the oocyst. 

 The sporozoites whether few or numerous, are naked gymnospores, 

 similar to those of the genus Eimeria amongst Coccidia. 



The Haemosporidia have been the object of extended studies on the 

 part of Labbe, many of whose statements, however, still require con- 

 firmation, especially with regard to the forms inhabiting cold-blooded 

 vertebrates, i.e. the genera Lankesterella, Karyolysus, and Haemoyreyarina. 

 It is asserted by him, with regard to the first two genera, that a trophozoite, 

 after growing to a certain size within a blood-corpuscle, becomes free in 

 the blood-serum, and that an isogamic conjugation takes place between 

 two perfectly similar free individuals ; and that then the zygote so 

 formed penetrates a second blood - corpuscle, or it may be a cell of the 

 spleen, liver, kidney, or bone-marrow, and forms a resistent cyst within 

 which it breaks up into sporozoites. A certain amount of scepticism has 

 grown up with regard to these statements, which are not in any way 

 confirmed by the recent observations of Hintze upon Lankesterella, and 

 receive no support from the analogy of what is known in other forms. 



