THE LOBOSA 87 



migrate towards the centre of the protoplasm. It is probably this 

 central party of nuclei that, with their surrounding protoplasm, 

 protrude from the pylome of the shell and give rise to the daughter 

 Arcella in the process of fission. 1 



In Centropyxis (Schaudinn [27]) the formation of secondary 

 nuclei previous to fission does not occur. When fission is about to 

 take place, a considerable portion of the protoplasm protrudes from 

 the pylome, assumes the inverted form of the parent, and develops a 

 shell. The nucleus remains in that part of the protoplasm which 

 at this stage only half fills the shell of the parent Centropyxis. When 

 the daughter shell is formed the nucleus increases to nearly double 

 its former size, the nucleolus dwindles in size, and numerous minute 

 chromosomes are formed. These changes are followed by the 

 formation of a spindle, the arrangement of the chromosomes in an 

 equatorial plate, and subsequently by nuclear division. One of the 

 nuclei thus formed passes into the daughter individual and the other 

 remains in the parent. 



While these changes in the nucleus are taking place, the 

 chromidial network divides into a great number of chromidia, which 

 collect round the two nuclei in equal proportions and pass with 

 them into the resultant individuals. 



Encystment. The formation of resting cysts occurs in Arcella, 

 Centropyxis, Nebela, Difflugia, and probably in all the other 

 Thecamoebida (Martini [16]). 



In Centropyxis, Schaudinn found that cysts are formed when 

 external conditions are unfavourable, such as in cases of desiccation, 

 scarcity of food, etc. In such cases the food particles, diatom 

 shells, excreta, a considerable proportion of the water, and any other 

 non-essential contents of the protoplasm, are ejected, while the 

 cytoplasm, with the contained chromidial network and nucleus, 

 contracts into a ball and is surrounded by a cyst- wall. 



At the end of encystment the cyst -wall disintegrates, the 

 protoplasm swells up to its former size, and the normal processes 

 of life are continued. It does not seem probable in this case 

 that encystment has any connexion whatever with the sexual 

 process. 



In Arcella, however, according to Hertwig (11), a reduction 

 in the number of the nuclei takes place, and it is suggested that 

 the process of conjugation may occur during this period of encyst- 

 ment, in a manner similar to that which occurs in ActinospJiaerium. 1 



In Difflugia urceolata (Zuelzer [35]) a process of encystment occurs 

 in the late autumn, and is accompanied by a destruction of a great 

 many of the old nuclei. Before the cysts rupture in the spring the 

 contents break up into a number of uninucleate secondary cysts, 

 but the history of the secondary cysts has not been followed. 



1 See Note, p. 93. 



