HISTORICAL SKETCH 



431 



all of the data at hand. He distinctly defined cell-division and free 

 cell-formation, and showed that what had been taken for the latter might 

 be regarded as a special case of the former. Nageli's conclusions were 

 supported by new evidence furnished by other investigators, who further 

 held that not only vegetative cells, but also those reproductive cells (in 

 thallophytes) which Nageli thought in some cases might be formed freely, 

 originate by a modified process of cell-division. It now seemed clear to 

 these men that cells arose only from preexisting cells, a conception which 

 had been emphasized by Remak (1841), and which Virchow (1855) 

 expressed in the dictum, omnis cellula e cellula. This dictum is frequently 

 employed today, but it is obvious that some qualification should be made 

 for the origin of cells in plasmodial masses. 



Fig. 230. — Figures of division of Tradescantia microsporocyte drawn by Wilhelm 

 Hofmeister (1848) many years before chromosomes were recognized as definite nuclear 

 units. 



Opinions concerning the origin of the nucleus and its role in cell-divi- 

 sion varied greatly among these workers, reliable observations being 

 insufficient to allow any definite conclusion. In 1841 Henle believed with 

 Schleiden that the nucleus was formed by the aggregation of "elemen- 

 tary granules," and that it was not constantly present. Better observa- 

 tions soon showed the falsity of this view. Von Kolliker (1845) asserted 

 that nuclear division precedes the division of the cell. Hofmeister, 

 studjdng embryo development in angiosperms, reported "that the nucleus 

 of the mother-cell divides into two; that one half of the contents of the 

 cell collects around each of the daughter-nuclei ..." (1847). It 

 was commonly believed at this time that the nucleus actually dissolved 

 just before cell-division, new nuclei being formed in the daughter cells; 

 but Hofmeister, although he described the dissolution of the nuclear 

 membrane and the nucleoli (1848), held that the main bulk of the nuclear 

 material actually separated into two masses. At this early date he 



