CHAPTER II 



CELL-DIVISION 



" Wo eine Zelle entsteht, da muss eine Zelle vorausgegangen sein, ebenso wie das Thier 

 nur aus dem Thiere, die Pflanze nur aus der Pflanze entstehen kann. Auf diese Weise ist, 

 weiingleich es einzelne Punkte im Korper gibt, wo der strange Xachweis noch nicht gelie- 

 fert ist, doch das Princip gesichert, dass in der ganzen Reihe alles Lebendigen, dies mogen 

 nun ganze Pflanzen oder thierische Organismen oder integrirende Theile derselben sein, ein 

 ewiges Gesetz der continmrlichen Entwicklung l:)esteht." ViRCHOW.^ 



The law of genetic cellular continuity, first clearly stated by Vir- 

 chow in the above words, has now become one of the primary data 

 of biology, and the advance of research ^s ever adding weight to the 

 conclusion that the cell has no other mode of origin than by division 

 of a preexisting cell. In the multicellular organism all the tissue- 

 cells arise by continued division from the original germ-cell, and 

 this in its turn arises by the division of a cell preexisting in the 

 parent-body. By cell-division, accordingly, the hereditary substance 

 is split off from the parent-body ; and by cell-division, again, this 

 substance is handed on by the fertilized egg-cell or oosperm to every 

 part of the body arising from it.^ Cell-division is, therefore, one of 

 the central facts of development and inheritance. 



The first two decades after Schleiden and Schwann ('40-'6o) were 

 occupied with researches, on the part both of botanists and of zool- 

 ogists, which finally demonstrated the universality of this process 

 and showed the authors of the cell-theory to have been in error in 

 asserting the independent origin of cells out of a formative blastema.^ 

 The mechanism of cell-division was not precisely investigated until 

 long afterward, but the researches of Remak ('41), Kolliker ('44), 

 and others showed that an essential part of the process is a division 

 of both the nucleus and the cell-body. In 1855 {I.e., pp. 174, 175), and 

 again in 1 858, R emak gave as the general result of his researches 

 the following synopsis or scheme of cell-division. Cell-division, he 

 asserted, proceeds from the centre toward the periphery. It begins 

 with the division of the nucleolus, is continued by simple constriction 

 and division of the nucleus, and is completed by division of the cell- 



1 Cellularpathologie. p. 25, 1858. '^ Cf. Introduction, p. 10. 



^ For a full historical account of this period, see Remak, Untersiuhungen iiber die Ent- 

 wicklung der IVirhelthiere, 1855, pp. 1 64-180. Also Tyson on the Cell-doctrine and Sachs's 

 Gcschichte der Botanik. 



6.^ 



