DIVISION OF CELLS iii 



division of an originally single and simple cell, the ovum or egg- 

 cell, and that there is no evidence of cells being formed 

 autogenously, like crystals out of their mother liquor, as 

 Schwann had supposed. The continued labours of many 

 investigators have confirmed Remak's conclusions, and it is 

 now an established fact, universally assented to, that every cell 

 arises from the division of a pre-existing cell, a generalisation 

 neatly summed up in Virchow's aphorism, Omnis cellula e 

 cellula. 



It was observed at a comparatively early date that the 

 nucleus took an important if not a predominant share in the 

 reproduction of cells by division, but the significance of this 

 fact was for a long time obscure, and it seemed of less im- 

 portance because it was long believed that there werq many 

 simple uni-cellular organisms destitute of a nucleus. Improved 

 methods of observation, however, have demonstrated the exis- 

 tence of a nucleus, or at least of the essential constituent of 

 a nucleus, chromatin, in many animal and vegetable cells in 

 which its existence was formerly denied, and there now remains 

 but a very small number of cases in which the presence of 

 chromatin has not been satisfactorily demonstrated. 



The part played by the nucleus assumed a new significance 

 after the discovery by Anton Schneider in 1873 of remarkable 

 form-changes undergone by the nuclei of tissue-cells at the 

 time of reproduction by division. Schneider's observations 

 were followed up by a host of observers, amongst whom the 

 names of Fleming, Fol, Ed. van Beneden, and Hertwig must 

 always occupy an honourable position ; and it has been demon- 

 strated that the phenomena first observed by Schneider are of 

 normal occurrence, not only in dividing tissue-cells, but also 

 in the germ-cells, the developing ova and spermatozoa of 

 multi- cellular animals, and in a large number of simple uni- 

 cellular animals, or Protozoa. 



The phenomena are generally known as karyo-kinesis, but 

 the word mitosis is also used to denote them, and it is, on the 

 whole, the more convenient term, though it must yield priority 

 to the other. 



As has been already said, division of the cell-body is always 

 accompanied by and generally preceded by division of the 

 nucleus. In certain, but comparatively rare, cases the nucleus 

 divides quite simply. It becomes elongated, and then dumb- 



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