DETAILS OF MITOSIS 



95 



nucleus with no trace of a centrosome. In this same form, as 

 described above, the ordinary vegetative mitoses are quite of the 

 metazoan type, with a persistent extranuclear centrosome. 



The hisiory of the chromatin in the mitosis of unicellular forms 

 shows some interesting modifications. In a considerable number of 

 forms a more or less clearly marked spireme-stage precedes the forma- 

 tion of chromosomes (diatoms, Infusoria, dinoflagellates, Euglypka)\ 

 in others, long chromosomes are formed without a distinct spireme- 

 stage (Noctiluca). It has been clearly demonstrated that in some 

 cases these chromosomes split lengthwise, as in Metazoa (Nocttlnca, 



Fig. 43. Mitosis in Par amoeba. [SCHAUDINN.] 



At the left, amoeboid phase, showing nucleus and " Nebenkorper." At the right, four stages 

 of division in the swarm-spores. 



diatoms, Actinophrys, probably in Euglypha) ; but in some cases they 

 are stated to divide transversely in the middle (Infusoria according 

 to Hertwig, Ceratium according to Lauterborn). These chromosomes 

 appear always to arise, as in Metazoa, through the linear arrangement 

 of chromatin-granules (Noctiluca^ Actinosphcenum, Euglena), which 

 themselves in many cases arise by the preliminary fragmentation of 

 one or more large chromatin-masses (e.g. in Noctiluca or Actinosphce- 

 riuni). In other forms no such linear aggregates are formed, and 

 direct fission of the chromatin-granules appears to take place without 

 the formation of bodies morphologically comparable with the chromo- 

 somes of such forms as Noctiluca. This is apparently the case in 

 Tetramitus, and Achromatium, other forms having a distributed 



