SARCODINA 215 



numerous nuclei show no differentiation amongst themselves, and 

 appear to be perfectly equivalent both in structure and function. 

 Chromidia may be present as a permanent cell-constituent in many 

 Amoebaea, such as Arcella, Difflugia, and the Foraminifera ; in other 

 cases they are formed temporarily, as extrusions from the nucleus, 

 during certain phases of the life-cycle, either as a preliminary to 

 reproduction or as a regulative process under certain physiological 

 conditions. 



The reproduction of the Sarcodina is effected either by binary 

 or multiple fission. Binary fission may be absent in some of the 

 larger, more specialized forms, as in many Foraminifera and Radio- 

 laria, but in most cases it is the ordinary " vegetative " method of 

 reproduction during the active trophic life of the organism. In 

 plasmodial forms it takes the form of plasmotomy (p. 100). Mul- 

 tiple fission or gemmation (sporulation) is in some cases the sole 

 method of reproduction ; in other case it is combined with binary 

 fission, and occurs only at certain crises in the life-cycle, in relation 

 to seasonal changes, or as a preliminary to syngamy. In this type 

 of reproduction the organism, breaking up rapidly into a large 

 often an immense number of minute individuals, is necessarily 

 put hors de combat as soon as the reproduction begins ; hence it is 

 not uncommon for the sporulation to take place within a cyst, 

 when a shell or protective envelope is not present, as in Amoeba 

 proteus (Scheel). The minute germs produced by sporulation may 

 be set free at once as swarm-spores ; or they may form a pro- 

 tective envelope or sporocyst, and be liberated as resistant spores 

 which are disseminated passively, and germinate when conditions 

 are favourable, as in parasitic forms and in the semi-terrestrial 

 Mycetozoa. 



The swarm-spores, whether produced directly by sporulation of 

 an adult or indirectly by germination of a spore, may be either 

 flagellulae or amoebulao. In many forms two types of sporulation 

 occur schizogony producing agametes, and sporogony producing 

 gametes. The agametes may be structurally or morphologically 

 distinguishable from the gametes. Thus in Foraminifera the 

 agametes are amoabulaBj the gametes. are flagellulse. In Radio laria 

 both alike are flagellulae, but the agametes produced in schizogony 

 the "isospores" are distinguishable from the gametes produced 

 in sporogony " anisospores." 



In this class syngamy takes place rarely between adult indi- 

 viduals ; but examples of this are seen in Actinophrys, where it takes 

 the form of karyogamy within a cyst (Fig. 71), and in Arcella (p. 148) 

 and Difflugia, where it takes the form of chromidiogamy between 

 free individuals, followed in Difflugia by encystment. In the great 

 majority of Sarcodina the syngamy is microgamous, and takes place 



