DIVISION OF CELLS in 



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 did not attract much 

 attention until the discovery by Anton Schneider in 1873 of 

 remarkable form-changes undergone by the nuclei of dividing 

 tissue-cells. 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 honour- 

 able position ; and it has been demonstrated that the 

 phenomena first observed by Schneider are of normal occur- 

 rence, 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 already been 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- 

 bell shaped. The strand connecting its swollen ends becomes 

 thinner, and finally breaks, so that two new nuclei are formed. 

 The cell-body becomes constricted in its middle and eventu- 

 ally is separated into two halves, each containing a portion 

 of the divided nucleus. Such a mode of nuclear division is 

 known as simple or amitotic. It occurs in some Protozoa, in 

 some leucocytes, and characteristically in certain cells forming 

 morbid growths. But in the great majority of cases the 

 process is of a far more complex kind, and it differs somewhat 



