10 BRITISH FRESHWATER HELIOZOA. 



fusion (it is uncertain which), the body then divides 

 into portions, each of which contains a single nucleus, 

 and these form " primary : ' cysts with silicious spicular 

 investments ; these each again divide into two, the 

 nucleus dividing by a process of mitosis ; the chromo- 

 somes are numerous, being estimated at from 130 to 

 150 in number; these secondary cysts behave like 

 gametocytes and undergo two nuclear divisions, in 

 each of which centrosomes are formed and one of the 

 two polar bodies is extruded, with a resting-stage 

 occurring between the processes ; the two gametes 

 thus formed from a primary cyst fuse into a zygote 

 containing a single fertilised nucleus ; a multiplication 

 of nuclei then takes place within the cyst ; and finally 

 an amoeboid multi-nuclear individual emerges. If 

 division or budding takes place without encystment, 

 no centrosomes are formed during the division of the 

 nuclei, the process being araitotic.* The animal may 

 also divide into various-sized, multi-nuclear portions 

 without any internal structural changes, the object 

 being apparently merely to reduce its size. 



The second group contains species which have the 

 nucleus placed centrally, and the processes of repro- 

 duction which have been observed in Actinophrys sol 

 are probably typical of those in the remaining species 

 of the group. 



Fission is most frequent (see Text-fig. 180), but copu- 

 lation or syngamy also occurs ; the latter process is 

 preceded by the formation of a gelatinous cell-wall 

 around two individuals ; the two nuclei then divide by 

 karyokinesis, one portion of each being extruded as a 

 polar body or reduction nucleus, the remaining portions 

 unite into a synkaryon and a complete fusion of the 

 cytoplasms takes place ; the result is a zygote which 

 subsequently, with karyokinesis of the synkaryon, 

 divides and forms two resting-spores from each of 

 which a young individual eventually emerges. 



* Bratier, A., ' Zeitschr. wiss. Zool.' Iviii (1894), p. 184. Sckaudinn, F., 

 ' Verb, deutsch. zool. Ges. Bonn/ vi (1896), p. 113. 



