160 JULIAN S. HUXLEY. 



The two nuclei meanwhile become round and very large, and 

 possess ou one side a large lenticular nucleolus. 



The next step in the cycle is for the associated couple, 

 while still in the liver-tube, to form a thick resistant coat 

 round itself : in so doing it becomes perfectly spherical, and 

 a process of concentration of cytoplasmic materials must take 

 place, as I have found none of these cysts with a diameter of 

 more than 115 im, and one only 85 /u across, the average being 

 about 100 ju. 



The formation of the cyst wall of necessit}' closes the 

 trophic periods, and sporogony now presumably begins. I 

 say presumably, for I have seen no spores, nor even any of 

 the preparatory nuclear divisions. Two cysts in the liver of 

 a particular host showed nuclei with central nucleoli emitting 

 chromatin (fig. 17) — a phenomenon very common in Protozoa 

 at the close of vegetative life : and I have found a number of 

 the usual type of cysts free in the gut. 



From these facts, and from analogy with other intestinal 

 Gregarines, we must suppose that after the formation of the 

 smooth cyst wall, the couples can be expelled from the liver 

 tubes (while those in the second trophic period remain in 

 place by virtue of their soft surface adhering to the similar 

 surfaces of the liver-cells), that they are then passed out by 

 the anus, and that it is only here, under the stimulus of the 

 changed conditions, that the processes leading to the pro- 

 duction of spores can take place. 



This being so, it is probable that infection is casual, the 

 spores or sporocysts being taken in with the food — as, 

 indeed, might have been deduced from the feeding habits of 

 An asp ides. The infection is usually heavy (text-fig. 3), 

 and frequently seems to be multiple, cysts, motile Gregarines, 

 and associated immobile forms being often found all in one 

 host. The proportion of infected hosts was over 50 per cent, 

 in the case of those that were captured by Mr. Smith in a 

 small pool on one of the mountain becks of Mt. Wellington ; 

 but in those he obtained from a larger piece of water, the 

 infection was nil — or at least no parasites were forthcoming 



