IN reply to Miss Lister's recent communication,1 I should like to raise the following points: 1. The statement concerning the spore-wall, p. 106 of my paper,1 is meant to apply particularly to Didymium nigripes var. xanthopus and not generally to other members of the Mycetozoa. The complete statement is as follows: “The spore of Didymium nigripes is rounded to oval in shape and approximately 10 to 12 μ in diameter. The spore membrane is comparatively thin, consisting of a single dark-coloured layer, 2 μ in width, and not of two layers as stated in Lister's Monograph.” 2. I do not wish to disregard the monospore cultures made by other investigators, but merely to point out that I have had difficulty myself. Indeed, I am of the opinion that there is a much greater possibility of obtaining satisfactory monospore cultures in Didymium nigripes var. xanthopus than there was in Reticularia (p. 126). On p. 99 I refer to Cayley's monospore cultures and express the hope that I shall be able to repeat my endeavour to isolate single spores, using her method. 3. On p. 129 I refer to the discovery of flagellate fusion in Didymium difforme by Cayley, but I do feel that such a conclusion should be supported by investigation of nuclear detail in stained preparations. 4. I have already apologised to Miss Cayley for having misunderstood her views concerning the connexion of sex segregation with meiosis, and am sorry that I was so much influenced by her diagram, which she admits is rather misleading.