* For details of the practical methods for testing viability see Appendix.

* Krigsmann (1926) described as wrinkled and then empty oocysts which had been for 17–23 hr. in ferment at 37° C., and he apparently used this condition as a criterion of excystation and perhaps naturally found it more rapid after successive action of pepsin and trypsin.

* Cf. Wenyon's (1926, p. 846) ‘E. canis’. There would seem to be little doubt that the dogs in the faeces of which he found these parasites had been having an extensive feed on infected rabbits. All the oocysts given in his Fig. 365 might belong to one of the four or five species of Eimeria of the rabbit, and Wenyon himself mentions their resemblance to E. stiedae and E. perforans.

* Dr Philpot, of the Biochemical Department, kindly provided me with this and some other solutions.

† Kofoid, McNeil & Kopac, (1931) concluded provisionally that the cysts of E. histolytica (and some other human intestinal parasites) had the properties of the group of keratinsGoogle Scholar. So far as I know, no conclusive tests have yet been made on the single wall of E. histolytica. It is interesting to note that in E. coli, Dobell (1938) has stated that the endocyst, which is comparatively thick and rigid, resembles chitin in composition and that its opening (before that of the thinner and elastic ectocyst) is brought about by a ferment secreted by the enclosed Entamoeba and not by purely mechanical means.

‡ I am indebted to Mr P. 'Espinasse for demonstrating this and to Dr J. R. Baker for lending his microscope fitted with polaroid disks.

* In spite of this one finds many unsuitable reagents recommended—in one laboratory 30% glucose was used. This has practically the same density (l·09) as the oocysts, and in it they would remain permanently suspended.

* I am grateful to Mr Medawar for providing me with quantities of this mixture and also with a 5 % solution of trypsin in Tyrode's solution, specially made up. He also gave me the chicks used in the experiments described below, as soon as they were hatched in his laboratory.

† Mr J. Z. Young kindly conducted these few feeding experiments on chicks and rabbits under his licence.

* I am grateful to Prof. Sir H. W. Florey for this very healthy 4-week-old rabbit reared under specially clean conditions in the Pathological Department by Mr Jesse Wheal. The mother was also free from coccidia.

* Temperature 35–36° C.

† Cf. p. 77.

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