1912.] On a Gregarine present in Mid- Gut of Ceratophylli. 35 



individuals, and the formation of the cyst wall and of the gametes succeed 

 each other very rapidly in this species. 



A period of about two months elapses between the time of infection and 

 the first appearance of cysts. This was ascertained by examination at 

 regular intervals of the series of larval, pupal, and adult fleas, which were 

 infected in the laboratory (see below, p. 36). Young larvae, recently hatched, 

 were collected on August 5, 1910, and placed, on August 8, in the debris 

 of a nest known to be infected with the Gregarine. During the fortnight 

 following August 10, 14 larvae were examined ; in all of them early 

 trophic phases of the parasite were present. On August 25, cocoons 

 were noticed ; they contained either pupae or scarcely formed adults. On 

 August 26, 27, and 30, young adult fleas were removed from the cocoons 

 and dissected, but only early vegetative phases of the Gregarine were 

 found. Other adult fleas from this series, dissected at intervals during 

 September, also exhibited trophic phases of the parasite. The first cysts 

 were observed on October 15, that is, more than nine weeks after the larvae 

 had been placed in the infective environment. 



The cysts are approximately spherical (fig. 10), and range from 110 to 

 185 /x in diameter.* When first formed they are translucent and grey, but 

 they become finally yellow or yellowish-brown in colour. A mucous portion 

 of the cyst envelope (i.e. the epicyst) appears to be absent, but the endocyst 

 is well developed. When freshly formed, and for a considerable time after- 

 wards, the endocyst is about 8 /a thick, but in very old cysts it has evidently 

 undergone condensation, becoming thinner (1 to 2 yu,), yellow, and less 

 translucent. The cyst envelope is plain, that is, not sculptured in any way. 

 The two individuals associated in each cyst give rise to gametes of apparently 

 similar form, but we have not investigated the gametes, nor have we observed 

 their fusion. 



Each cyst soon contains numerous spores, among which are one or more 

 masses of residual protoplasm, situated in the centre of the cyst. Each spore 

 is oval, and is 11 to 12 /a long and about 7 /a broad (fig. 11). The epispore 

 is closely applied to the endospore and exhibits two polar thickenings. Each 

 spore, when ripe, contains eight sporozoites and a small amount of axial 

 residual protoplasm. 



Cysts — 1 to 1-1 in number, in different individuals — occur lying freely in 

 the mid-gut of the flea. They cannot escape entire from the digestive tract, 



* We have occasionally seen small cysts, about 50 n in diameter, but these did not 

 contain spores. Probably each cyst resulted from the encystment of a single individual, 

 and none of them would have developed further. The contents of two were examined, 

 and were found to have degenerated. 



D 2 



