32 MORPHOLOGY AND CULTURE OF MICROORGANISMS 



a dumb-bell. Soon the partition stops completely, the filaments of 

 the contracted part of the nucleus break up and the two daughter cells 

 appear separated by a partition. The two nuclei whose filaments have 

 been sectioned by the partition are not slow in recovering their in- 

 tegrity (Fig. 6, b). 



We find in the Amoeba mucicola (Fig. 21, A) a much more char- 

 acteristic mitosis, though more primitive. The nucleus of this amoeba 

 when at rest is made up of a nuclear fluid surrounded by a membrane 

 in which are a large karyosome and some small grains of chromatin 

 localized on the periphery (Fig. 21, A). In the center of the karyosome 

 is a small chromophilic centriole. The prophase begins by the elonga- 

 tion of the karyosome to a rod-shaped body (Fig. 21, A, b) which then 

 transforms itself into a dumb-bell (Fig. 21, A, c). The centriole 

 also elongates and becomes constricted in the center (Fig. 21, A, d). 

 At the same time an achromatic spindle appears all about the con- 

 stricted region of the karyosome in the middle of which the grains of 

 chromatin arrange themselves peripherally to form an equatorial 

 plate, but there_is no differentiation of this chromatin into two 

 chromosomes (Fig. 21, A, c, d). In the metaphase the karyosome 

 and the centriole divide into two polar masses (Fig. 21, A, e, /), the 

 equatorial plate separates into two plates which, in the anaphase, 

 emigrate to the poles (Fig. 21, A, g) drawn by the centrioles. In the 

 telophase the spindle elongates, disappears, and the two nuclei are 

 formed at the poles (Fig. 21, A, i, k). The nuclear membrane exists 

 during the entire phenomenon. 



In other microorganisms (Amoeba, Flagellata, Euglena) is found a 

 similar mitosis except that the chromatin distributed in the resting 

 nucleus as a network or as rod-shaped bodies forms an equatorial plate 

 made up of true chromosomes (Fig. 21, B, C). 



Another form of mitosis, promitosis, is characterized by the fact 

 that the centriole is included in the karyosome, by the persistence of 

 the nuclear membrane, and by the simultaneous division in the meta- 

 phase of the karyosome and of the chromatin gathered in an equatorial 

 plate. 



Between promitosis and metamitosis are a series of intermediate 

 forms. In the Pelomyxa palustris, for example, the centriole while 

 remaining intranuclear is able to separate itself from the karyosome 

 (Fig. 22, A } a). The prophase here begins with the usual division of 



