THE SARCOBINA 



221 



and produce within the cyst a number of gametes in the same manner 

 as the agametes already described, but with the following differ- 

 ences of detail : the principal nucleus degenerates as soon as the 

 chromidia are formed ; the number of secondary nuclei produced is 

 much larger, and the gametes are much smaller than the agametes ; 

 and the cytoplasm of the parent is entirely used up in their forma- 

 tion. The gametes are ultimately set free from the cyst as amoe- 

 bulse, and pair ; the zygote grows into the adult form of the amoeba. 

 Such a cycle has recently been described by Popoff (264) for a 

 species named by him Amoeba minuta ; the gametes in this species 

 are isogametes, without any sexual differentiation as in Arcella. 

 This type of life-cycle is probably very common in many amoebae, 



FIG. 87. Amoeba albida : autogamy in the encysted condition ; drawn in outline, 

 with nuclear details only. A, Encysted amoeba ; B, the nucleus of the 

 amoeba divides unequally into a larger vegetative and a smaller generative 

 nucleus ; the vegetative nucleus, as seen in the subsequent figures, travels 

 to the surface of the cyst, degenerates, and disappears ; the generative nucleus 

 gives rise to the gamete-nuclei ; G, incomplete division of the generative nucleus ; 

 D, one half of the generative nucleus is budding off two reduction-nuclei 

 (on the right) ; E, four reduction-nuclei have been budded off, two from 

 each pole of the incompletely divided generative nucleus ; F, the reduced 

 generative nucleus completes its division ; the four reduction-nuclei are 

 degenerating ; 0, the two pronuclei far apart ; H, the two pronuclei coming 

 together ; /, the pronuclei fusing. After Nagler (95). 



with specific differences of detail in different cases, of which the 

 most important are, that in some cases, probably, the nucleus 

 divides to form the gamete-nuclei, instead of becoming resolved 

 into chromidia, and that autogamy within the cyst may occur, 

 instead of free gametes being formed, as A. albida (Fig. 87). 

 According to Nagler (95), autogamy of this type is characteristic 

 of all amoebae of the Umax-group ; in such cases only two gamete- 

 nuclei are formed in the cyst, which after going through reducing 

 divisions fuse to form a synkaryon. The zygote then leaves the 

 cyst and begins a fresh vegetative cycle. 



A different type of life-cycle is exemplified by that which Schepo- 

 tieff has described in the case of a marine amoeba identified by him 

 as A. ftava. In this case also the ordinary vegetative form is a 



