62 



CELL-DIVISION 



Hacker has called attention to the striking fact that plurivalent 

 mitosis is very often of the heterotypical form, as is very common in 

 the maturation mitoses of many animals (Chapter V.), and often 

 occurs in the early cleavages of Ascaris ; but it is doubtful whether 

 this is a universal rule. 



4. Mitosis in the Unicellular Plants and Animals 



The process of mitosis in the one-celled plants and animals has a 

 peculiar interest, for it is here that we must look for indications of 



Fig. 27. Mitotic division in Infusoria. [R. HERTWIG.] 



A-C, Macronucleus of Spirochona, showing pole-plates. D-H. Successive stages in the 

 division of the micronucleus of Paramaecium. D. The earliest stage, showing reticulum. G. Fol- 

 lowing stage ("sickle-form") with nucleolus. E, Chromosomes and pole-plates. F. Late ana- 

 phase. H. Final phase. 



its historical origin. But although traces of mitotic division were 

 seen in the Infusoria by Balbiani ('58-'6i), Stein ('59), and others 

 long before it was known in the higher forms, it is still imperfectly 

 understood on account of the practical difficulties of observation. 

 Within a few years, however, our knowledge in this field has rapidly 

 advanced, and we have already good ground for some important 

 conclusions. 



Mitotic division has now been observed in many of the main divi- 

 sions of Protozoa and unicellular plants; but in the present state of 

 the subject it must be left an open question whether it occurs in all. 



