THE SPOROZOA 263 



From the facts it would appear at first sight as if the infection of 

 the young ticks was a case of true hereditary infection, parallel to the 

 "pe"brine" disease of the silkworm. But it is quite possible that the 

 tick-embryo acquires the infection secondarily from the blood it absorbs 

 in the egg, and it does not follow that the parasitic germs pass through 

 the ovum itself as in Glugea. Until something is known of the stages 

 of the parasite within the tick, it is not possible to decide whether this 

 is a case of true hereditary infection or not. 



The number of instances in which intermediate hosts have been 

 demonstrated for Haemosporidia has been increased so steadily 

 by recent researches that many authorities are inclined to the 

 belief, to which expression has recently been given by Borner, that 

 for all species of Haemosporidia there is some blood-sucking animal 

 which is the agent in the dissemination of the parasites, and that 

 where no intermediate host is known, it merely remains to be 

 discovered. 



There are, however, many grounds against believing that an 

 intermediate host occurs in all cases. First, on general grounds, 

 if the modern conception of the Haemosporidia as forms closely 

 allied to Coccidia, adapted to parasitism upon blood -cells, be 

 correct, it is reasonable to suppose that the ancestors, at least, of 

 the group under consideration were at first without any special 

 means of dissemination other than the resistent spores and cysts 

 found in Coccidia and Sporozoa generally ; and if this be admitted, 

 it becomes further highly probable that representatives of these 

 primitive forms will be found to exist at the present day. 

 Secondly, empirical grounds are not wanting to support these 

 conclusions, although decisive experimental proof is lacking as 

 yet. In a great many instances amongst the Haemosporidia of 

 the lower Vertebrata, sporogony as well as schizogony occurs 

 in the Vertebrate host. In the case of the Lankesterella of the 

 frog, Hintze has shown that the motile zygote leaves the blood to 

 encyst in an epithelial cell of the gut, and that the resistent cyst 

 so formed passes out with the faeces. We find here, therefore, 

 just those conditions for disseminating the parasites which are 

 most typical of Sporozoa generally. It is highly probable that the 

 infection of the frog by Lankesterella is a casual one, brought about 

 by the frog swallowing cysts of the parasite accidentally, and this 

 conclusion is supported by the fact that, according to Hintze's 

 observations, frogs living in pools and confined spaces are especially 

 liable to the infection, while those from rivers and large areas of 

 water are almost entirely free from it. 1 



There is therefore a very strong case in favour of the view 



1 Compare th very similar case of Lithobius from different localities ai regards 

 infection with Coccidium, above, p. 221. 



