AMITOSIS 49 



protozoan Euglypha. In the diatom Surirella it enters the nucleus 

 during prophase. In the Fungi and many Protozoa the centro- 

 some normally appears only at division. 



Another type of abnormality is the extreme reduction in the size 

 of the spindle relative to the nucleus. In this type, if the spindle is 

 still distinguishable, it lies to one side of the dividing nucleus, as in 

 Aggregata eherthi (Fig. lo). This organism demonstrates two other 

 peculiarities of mitosis. It undergoes a process of " multiple 

 nuclear division " during spore and germ-cell formation. In other 

 organisms the relationship of this process to true mitosis is still 

 obscure. Here the chromosomes do not show the high degree of 



Fig. io, — Diagram of mitosis in haploid nuclei of Protista. The 

 spindle is shown by broken lines, the nucleoli in outline and 

 the centrosomes by solid circles sometimes surrounded by 

 " centrospheres." Cryptomonas (Cryptomonadina) : no visible 

 centrosome, cylindrical spindle within the nucleus. Mono- 

 cystis (Eugregarina, w =4), progamic mitosis : centrosome 

 plate-like at metaphase {M), invisible at anaphase {A). Aggre- 

 gata (Coccidia, w = 6), sporogonial mitosis : two chromo- 

 somes shown at metaphase, one at later stages. Spindle outside 

 nucleus. Greatest spiralisation at telophase (T). Surirella 

 (Diatomeae) : centrosome enters nucleus and spindle develops 

 inside it during prophase. (After Belar, 1926.) 



contraction characteristic of metaphase. They retain the length 

 and appearance normally seen at early prophase, contracting further 

 at anaphase. Divisions follow one another rapidly, however, and 

 this further contraction is not always completed, so that the ends of 

 two chromosomes which are longer than the rest do not separate. 

 In this way daughter nuclei are constituted which run into one 

 another and suggest the appearance of simple fission of a nucleus — 

 what has always been called " amitosis." None the less the actual 

 longitudinal splitting of the chromosomes can be seen with the 

 utmost clearness, and in later divisions this separation is completed 

 (v. infra). 



These examples indicate the wide range of structure and 



