MODIFICATIONS OF MITOSIS 



67 



(Cf. pp. y?, and 221.) Richard Hertwig's studies on Infusoria and 

 those of Lauterborn on Flagellates indicate that here also no longitu- 

 dinal splitting of the chromatin-threads occurs and that the divi^sion 

 must be referred to the individual chromatin-granules. Ishikawa de- 

 scribes a peculiar longitudinal splitting of chromosomes in Noctihica, 

 but Calkins' studies indicate that the latter observer has probably mis- 

 interpreted certain stages and that the division probably takes place in 

 a somewhat different manner. A typical spireme and chromosome- 

 formation has also been described by Lauterborn in the Diatoms ('93). 



In none of the foregoing cases does the nuclear membrane dis- 

 appear. In the gregarines, however, the observations of Wolters 

 ('91) and Clarke ('95) indicate that the membrane does not persist, 

 and that a perfectly typical mitotic figure is formed. 



To sum up : The facts at present known indicate that the unicellu- 

 lar forms exhibit forms of mitosis that are in some respects transi- 

 tional from the typical mitosis of higher forms to a simpler type. 

 The asters may be reduced (Rhizopods) or wanting (Infusoria); the 

 spindle is typically formed inside the nucleus, either by division of an 

 intra-nuclear " nucleolo-centrosome " {Eicglena, Amceba), or possibly 

 by rearrangement of the chromatic substance without a differentiated 

 centrosome (.?micronuclei of Infusoria). In every case the essential 

 fact in the history of the chromatin is a division of the chromatin- 

 granules ; but this may be preceded by their arrangement in threads 

 or chromosomes {Euglypha, Diatoms) or may not {Actinosphcenimi). 

 These facts point toivards the conclusion Jhat centrosome, spindle, and 

 chromosomes are all secondary differentiations of the primitive nuclear 

 staictiire, and indicate that the asters and attraction-spheres may be 

 historically a later acquisition developed in the cytoplasm after the dif- 

 ferentiation of the centrosome. 



5. Pathological Mitoses 



Under certain circumstances the delicate mechanism of cell-division 

 may become deranged, and so give rise to various forms of patholog- 

 ical mitoses. Such a miscarriage may be artificially produced, as 

 Hertwig, Galeotti, and others have shown, by treating the dividing- 

 cells with poisons and other chemical substances (quinine, chloral, 

 nicotine, potassic iodide, etc.). Pathological mitoses may, however, 

 occur without discoverable external cause ; and it is a very interest- 

 ing fact, as Klebs, Hansemann, and Galeotti have especially pointed 

 out, that they are of frequent occurrence in abnormal growths such 

 as cancers and tumours. 



The abnormal forms of mitoses are arranged by Hansemann in two 



