MITOSIS 99 



sidered (Fig. 56). The resting nucleus consists of a fairly thick membrane 

 and a large central karyosome round which are arranged a series of 

 granules. When nuclear division, preparatory to division of the amoeba, 

 commences, the nuclear membrane becomes thin and the karyosome 

 fragments into a number of fine granules, while those which surround 

 the karyosome disappear. Those originating from the karyosome run 

 together to form larger granules, which become arranged as a long-coiled 

 chain of beads which, decreasing in length, finally occupies the equator 

 of the nucleus as a ring. The linin network of the nucleus now shows 

 indications of spindle-fibre formation and the nuclear membrane dis- 

 appears. The spindle, which has rounded ends and no centrosomes or 

 asters, becomes slightly elongated, while the chromosomes, sixteen in 

 number, which are arranged as a ring round the equator of the spindle, 

 divide so that two rings of daughter chromosomes are formed. These 

 separate from one another as the spindle itself becomes greatly drawn 

 out. Finally, each ring of daughter chromosomes which has moved to 

 the end of the spindle is broken up and a nuclear membrane is formed. 

 The daughter nucleus is at first flattened, but gradually increases in size, 

 and, with reconstruction of the karyosome, assumes the characters of 

 the original parent nucleus. Before this stage is reached the amoeba, 

 which has become elongated, is divided by constriction into two parts. 

 In this division there are no granules which could be interpreted as 

 centrioles at the apices of the spindle, nor was it possible to discover any 

 indications of a centrodesmose, so that it would appear that centrosomes 

 and centrioles are completely absent. 



The division of the nucleus of Entmnoeha histolytica, as seen in the 

 encysted forms, is of a similar type, but the nuclear membrane remains 

 throughout the process (Fig. 57). The earliest stage appears to be the 

 division of the minute central karyosome. The two daughter karyosomes 

 separate, while a spindle forms between them. On the equator of the 

 spindle, which is surrounded by the elongating nuclear membrane, appear 

 a ring of chromosomes in an equatorial plate. These divide to form 

 daughter chromosomes, which pass towards the poles of the elongating 

 spindle in an irregular manner. According to Kofoid and Swezy (1924o, 

 1 925) the chromosome number is six. As the spindle elongates the daughter 



1. Usual type of amoeba : nucleus with large central karj'osome surrounded by granules. 



2, 3. Karyosome breaking up into granules. 

 4. Chromatin arranged as irregular loop. 



."). iJi.sappearance of nuclear membrane : spindle with equatorial plate of chromosomes. 



()-9. Division of chromosomes to form daughter plates, which pa.ss to the poles of the elongating 



spindle. 

 1(1. Ciiinmencing division of amoeba. 

 II. Disappearance of spindle, reconstruction of nuclear membrane, and commencing reconstrue- 



tion of karyosome. 12. Encystr d amoeba. 



