28 . 



\. /t,j>ro< fi/rt ion of the cell. For some time after the discov- 

 ery of the cell as a unit in the formation of animal and plant 



in isms it was supposed that cells arose by growth from a 

 formlo.- germ substance, the so-called cytoblastema. A certain 

 similarity was traced between this and the process of crystalliza- 

 tion, whcn-in the cells were made to correspond with the crystals 

 and the cytoblastema with the mother liquid. Thanks, however, 

 to the observations and researches of Mohl, Nageli, and others, 

 the conception was arrived at that cells arise only by division 

 directly from other cells, which fact Virchow expressed in these 

 words, " Omnis cellula e cellula." Later, on the basis of further 

 investigation, the conception was changed to " Omriis nucleus e 

 nucleo." The increase of cells takes place by cell division r 

 which may be of two kinds, distinguished by the behavior of 

 the nucleus during division. We speak of a direct (amitotic) 

 and an indirect (mitotic] division. 



DIRECT DIVISION (AMITOSIS). 



By direct division the nucleus is separated into two daughter 

 nuclei by constriction, without any further important changes 

 in its structure being manifest. This kind of division is riot 

 common, and seems in certain cases to be the result of abnormal 

 processes ; for often the nuclear division is not accompanied by 

 a division of the cell. In this way multinucleated cells arise 

 (e. g., giant cells). There is ground for the belief that direct 

 division is a process which no longer tends to the physiological 

 increase of cells, but represents a degeneration (Flemming). 

 We find amitotic division especially in the lower animals. 

 more particularly in the protozoa; but it may occur also in 

 higher animals, along with indirect division in many leucocytes, 

 cartilage cells, decidual cells, surface epithelial cells of the 

 urinary bladder, etc. 



INDIRECT DIVISION (MITOSIS, KARYOKINESIS i . 



This division is characterized by a whole series of phenomena 

 in the nucleus and protoplasm, while in consequence of a solu- 



