POLYMORPHISM AND LIFE-CYCLES 179 



the opposite ends of a diameter of the circular body, and an irregular 

 ring of chromidia forming a dense chromidial net. Under certain 

 conditions Arcella becomes encysted, forming a spherical oyst 

 with a tough impervious membrane within the shell, closing the 

 mouth of it. 



The adult Arcella reproduces itself by a variety of methods, 

 which, however, may be reduced to two principal types : binary 

 fission, producing daughter-individuals (Arcellce) of approximately 

 equal size ; and gemmation, producing small amcebulae such as have 

 been described above as the starting-point of the ontogeny. The 

 production of the amcebulae may or may not be in relation to 

 syngamy, which, when it occurs, may be of one or the other of two 

 distinct types karyogamy between amcebulae, or chromidiogamy 

 between adult Arcellce. 



Binary fission (Fig. 80, D G) is the ordinary type of reproduction 

 during the " vegetative ' life in the summer months, when the 

 animal is actively feeding, growing, and reproducing itself. In 

 the process of binary fission, the two nuclei divide by a form of 

 karyokinesis (Fig. 57, p. 110). A quantity of the body-protoplasm 

 streams out through the mouth of the shell, together with some of 

 the chromidia, and one of the two daughter-nuclei of each pair also 

 passes out of the shell. The daughter -Arcella thus formed secretes 

 for itself a new shell, and separates from the parent-individual, 

 which retains the old shell. Thus in binary fission both nuclei and 

 chromidia take part, the former dividing by mitosis, while the latter 

 are subjected to a roughly equal partition. 



The ordinary binucleate form of Arcella may become multi- 



FIG. 80 continued: 



All the figures below the level of D represent reproduction by gemmation : 

 those to the left are reproductive processes not combined with syngamy ; 

 those on the right show the methods of syngamy. 



H, Formation of secondary nuclei and buds which are liberated singly 

 from the parent as amcebulae (a.). 



/, Rapid bud-formation, leading to almost the whole protoplasm of the 

 parent being used up to form them. 



J, Bud-formation external to the shell ; the protoplasm has streamed out, 

 leaving only a small residual portion, containing the primary nuclei, in the 

 shell ; the extruded protoplasm producing buds with formation of secondary 

 nuclei. 



K, L, Formation of gametes and karyogamy : K, formation of macramoeb* 

 (?);!/, formation of micramoebse ( <? ) ; the gametes ( ? and $ ) pass out of 

 the shell and copulate ( $ ) to produce the zygote or amcebula (a.). 



M Q, Chromidiogamy : M, two Arcellce coming together ; N, the proto- 

 plasm, with the chromidia and degenerating primary nuclei, of the one passes 

 over into the shell of the other ; 0, after intermingling of the chromidia, the 

 protoplasm becomes equally distributed between the two shells ; P, the 

 chromidia give rise to secondary nuclei ; Q, buds (amoebulse, a.) are formed 

 and liberated. 



Other letters : n., nucleus ; w. 1 , primary nucleus ; n. 2 , secondary nucleus; 

 chr., chromidia ; sh., shell ;"o, mouth of shell ; a., amoebulse. 



Modified from a diagram by Swarczewsky. 



