32 . THE HELIOZOA 



formed from the products of the division of a single primary cyst, 

 fuse again into a single zygote, their pronuclei uniting to form a 

 single fertilised nucleus. After this process is completed, a 

 membranous or gelatinous layer is formed within the siliceous 

 cyst, which Hertwig compares to the yolk-membrane so frequently 

 formed by fertilised ova. A multiplication of nuclei now occurs 

 within the cyst ; the creature becomes amoeboid, and emerges. 

 After emergence, individuals with a single nucleus are not very 

 rare, so that the amoeboid young may possibly sometimes divide ; 

 but the process has not been observed. 



If the foregoing account be correct, we have in Actinosphaerium 

 the only case in the whole animal kingdom in which self-fertilisa- 

 tion is shown to be of normal occurrence. There are, however, 

 several points to be considered before this view can be adopted 

 without qualification. Brauer (2) asserts that the formation of 

 Hertwig's " primary " cysts is preceded by a fusion of nuclei, so 

 that the nucleus of each primary cyst is really formed from two 

 resting nuclei, confirming the view put forward by Anthon Schneider 

 in 1877 ; and Hertwig admits that there is a considerable body 

 of evidence in favour of this view, though if such a fusion takes 

 place it must be a very rapid process, affecting all the nuclei 

 in the body simultaneously ; otherwise its occurrence must have 

 been frequently witnessed by an observer so skilful and patient 

 as Professor Hertwig. The frequent occurrence of plastogamy 

 between adult individuals makes it very possible that all the nuclei 

 in the same body may not be of the same origin ; and therefore 

 the formation of the primary cyst- nuclei by the fusion of two 

 others might, in many cases, at least, mean the fusion of nuclei 

 originally produced in different individual bodies (Schaudinn). 

 Such a preliminary fusion of the nuclei of gametocytes, which 

 separate before giving off polar bodies and finally fusing to form 

 a zygote nucleus, has been observed in Spirogyra (cf. Klebahn [11], 

 quoted by Hertwig) ; and a process of a similar kind a fusion 

 of gametocyte nuclei before the extrusion of polar bodies, the polar 

 bodies being only given off after division of the fertilised zygote 

 appears to occur in some desmids (Closterium, Klebahn [11]). If, 

 therefore, we can believe that an individual before encystment has 

 normally exchanged some of its nuclei for those of another indi- 

 vidual during an antecedent plastogamy, and that a fusion of nuclei 

 in pairs takes place before the formation of the "primary" cysts, 

 the nuclear history of Actinosphaerium will not be without parallel ; 

 but there is direct evidence that normal encystment may occur 

 without plastogamy, since Hertwig has succeeded in keeping an 

 isolated individual under control through the entire period from 

 " hatching " until the production of normal, fertile cysts. Again, 

 all observers are agreed that plastogamy is not necessarily followed 



