Lefevre — The Advance of Zoology in the Nineteenth Century. 97 



In 1833 the English botanist, Eobert Brown, saw in certain 

 plant structures that each cell contained a small circular spot 

 which he called the nucleus. Five years later, in 1838, Schlei- 

 den proposed the generalization that a nucleus was an uni- 

 versal elementary organ in plant bodies, and in 1839 the 

 doctrine was extended by Schwann to animal bodies. The 

 theory is hence commonly known as the Schleiden and 

 Schwann Cell-theory. 



At first the wall or membrane of the cell was considered 

 to be the most important part of the vesicle, while the sub- 

 stance contained within the wall, to which Von Mohl gave the 

 name protoplasm in 1846, was either overlooked or regarded 

 as a waste-product. Through a series of researches, mainly 

 by Bergmann, Kolliker, Bischoff, Cohn, de Bary and Schultze, 

 it was definitely proven that the cell-contents, not its walls, 

 is the seat of the vital functions, by showing that some cells, 

 as the white blood-cells, are merely naked masses of protoplasm. 

 It was then further demonstrated that the presence of a 

 nucleus was practically universal, and the cell came to be rec- 

 ognized as a" mass of protoplasm containing a nucleus," a 

 definition which is accepted to-day. The word cell is there- 

 fore a misnomer, but it has been perpetuated as an historical 

 survival in spite of numerous attempts to supplant it. 



Schleiden and Schwann believed that cells might arise in 

 the body by a process of crystallization out of a formative 

 substance, termed " cytoblastema." It was not until years of 

 careful research had elapsed that it was finally settled that new 

 cells are only produced by division of pre-existing cells, and 

 in 1855 Virchow announced his famous aphorism, omnis 

 cellula e cellula. This conclusion now rests upon an irre- 

 fragable basis. 



The mechanism of division, however, was little understood 

 at first, although it had been shown as early as 1841 by Remak 

 and Kolliker that both the nucleus and the body of the cell 

 divide. The division was supposed at that time to take place 

 by simple constriction, first of the nucleus into two parts, and 

 then of the cell-body into two, each containing one of the 

 daughter-nuclei. It was not until 1873 that the process was 

 shown to be of a far more complicated nature and to in- 



