THE LOB OS A 73 



The process of fission is usually preceded by division of the 

 nucleus, or in the binucleate Amoebae of both nuclei. 



In some species (Amoeba binucleata and Paramoeba eilhardi, 

 Schaudinn (Figs. 4 and 5), A. Umax, Vahlkampf, and Amoeba proteus, 

 Awerinzew) the division of the nucleus shows some of the charac- 

 ters of ordinary mitosis. The chromatin is collected together into 

 a large number of short chromosomes arranged in an equatorial 

 row. They then divide and travel in two equal parties along 

 faintly stained and nearly parallel lines, supposed to be of 

 the nature of linin, towards the opposite poles of the nucleus, 

 where they unite to form the chromatin network of the daughter 

 nuclei. The threads of the figure do not always converge at the 

 poles to a focal point, and as a general rule it is doubtful whether 

 structures exactly similar to the centrosomes of the metazoan nuclei 

 occur. Centrosomes, however, have been described and figured 

 in the division of the nuclei of Pelomyxa (Bott [2], Fig. 11, a). 



Notwithstanding the evidence of a primitive kind of mitosis in 

 the division of the nuclei in these and other species, the division of 

 the nucleus of Amoeba crystalligera, of A. hyalina, of Dactylosphaera 

 polypodia, and of Endamoeba coli (Schaudinn) is amitotic. 



In Pelomyxa and Trichosphaerium fission usually consists in the 

 pinching off of globules from the body, each containing a few 

 nuclei. These globules rapidly assume the characters of the 

 parent; they increase in size and the number of the nuclei is 

 .augmented. This process may be regarded as a case of unequal 

 fission or of gemmation, but it appears to differ from the equal 

 fission of some species of Amoeba in the respect that antecedent 

 division of the nuclei is not an essential preliminary to division of 

 the cytoplasm. In a large multinucleate form of Amoeba proteus, 

 Stole (31) has found that division of the nuclei may or may not 

 take place before fission ; and in some cases some of the nuclei 

 divide and others do not before an act of fission. 



Encystment and Spore-Formation. It is now known that many 

 of the Gymnamoebida periodically undergo a process of encystment 

 in which the pseudopodia are withdrawn, the body becomes more 

 or less spherical, and one or more tough membranes are formed 

 which entirely surround and protect the animal. In some cases 

 these cysts appear to be of the nature of resting cysts (Amoeba 

 Umax, Vahlkampf [33]), the organism emerging from the broken 

 cyst-wall with the same characters it possessed previous to cyst- 

 formation. The formation of resting cysts probably occurs in all 

 the Thecamoebida. In many cases, however, the encystment is 

 accompanied by complicated nuclear changes and divisions followed 

 by division of the cytoplasm, and a swarm of minute spores that 

 .are often very different in character from the parent form are 

 hatched out when the cyst-wall breaks down. 



