148 THE PROTOZOA 



(a) Karyogamy. The body of an Arcella gives rise by multiple 

 gemination to a number of amoebulae, each containing a secondary 

 nucleus derived from the chromidia, while the primary nuclei of 

 the parent-form degenerate (Fig. 80). The number and size of 

 the amoebulae vary, however, in different individuals. In one 

 Arcella the number is less and the amoebulae are larger, eight or 

 nine macramcebce being produced. In another the amoebulae are 

 more numerous and smaller, about forty micramcebce being formed. 

 In either case the amoebulae swarm out of the parent-shell and are 

 the gametes. A micramoeba copulates with a macramoeba, the 

 two fusing completely to form a zygote with a synkaryon. The 

 amoeboid zygote thus produced is the starting-point in the growth 

 and development of an Arcella (Fig. 80, A). 



In this example the karyogamy is a case of microgamy, which, 

 like other such cases, precedes a period of growth and follows a 

 period of active reproduction. It is possible that the syngamy 

 of the gametes is preceded by reducing divisions of the nuclei of 

 the amoebulae, but no such reduction has been observed in Arcetta. 

 In Foraminifera (p. 235), in which the syngamy is perfectly isog- 

 amous, each secondary nucleus formed from the generative 

 chromidia divides twice to form the gamete-nucleidivisions 

 doubtless to be regarded as reducing divisions. In Centropyxis, 

 according to Schaudinn (131), amoebulae, all of the same size, are 

 produced as in Arcetta, by formation of secondary nuclei ; but in 

 some broods each amoebula divides into four micramoebae (micro- 

 gametes), while hi other broods the amoebulae remain undivided as 

 macramoebae (macrogametes) ; copulation then takes place between 

 two gametes of different size. 



(6) Chromidiogamy (Fig. 80, M Q). Two ordinary adult Arcettas 

 come together and apply the mouths of their shells. The proto- 

 plasm of one individual flows over almost entirely into the other 

 shell, taking with it both chromidia and primary nuclei, only so 

 much protoplasm being left in the one shell as suffices to hold the 

 two shells together. The primary nuclei now degenerate, and the 

 chromidia derived from each conjugant break up into a fine dust 

 of chromatin-particles and become intimately commingled. When 

 this process is complete, the protoplasm with the chromidia 

 becomes again distributed between the two shells, and the two 

 conjugants separate. Then hi each individual secondary nuclei 

 are formed from the chromidia, and by a process of multiple gem- 

 mation a number of uninucleate amoebulae are formed which swarm 

 out of the shell, and, like the zygotes resulting from karyogamy, 

 become the starting-point of a new Arcetta. 



Thus chromidiogamy is here a case of macrogamy which, like 

 other similar cases, follows a period of growth and precedes a 



