430 INTRODUCTION TO CYTOLOGY 



None of the observers named above knew how the cells multiplied. 

 In his Beitrdge zur Phylogenesis, which dealt largely with the origin of 

 cells, Schleiden set forth his theory of "free cell-formation," which was 

 essentially as follows. In the general cell contents or mother liquor 

 ("cytoblastema") there are formed, by a process of condensation, certain 

 small granules (later called "nucleoli" by Schwann). Around these 

 many other granules accumulate, thus forming nuclei ("cytoblasts"). 

 Then, "as soon as the cytoblasts have attained their full size, a delicate 

 transparent vesicle appears upon their surface." This vesicle in each 

 case enlarges and forms a new cell, and, since it arises upon the surface 

 of the cytoblast (nucleus), "the cytoblast can never lie free in the interior 

 of the cell, but is always enclosed [i.e., imbedded] in the cell wall. ..." 

 Cell-formation was thus regarded as endogenous ("cells within cells") 

 rather than the result of cell-division. This erroneous idea was adopted 

 by Schwann, and although it detracted little from the value of his 

 histological work, it required correction before cytology could develop 

 further along progressive lines. 



Cell-multiplication. — The cell theory as outlined above was at 

 once adopted as a fundamental proposition in biological research, though 

 in certain of its aspects it underwent considerable modification as knowl- 

 edge increased. It was especially desirable to clear up the question of 

 cell origin, and to this task a number of able observers addressed them- 

 selves. Among these should be mentioned Hugo von Mohl (1805-1872), 

 Wilhelm Hofmeister (1824-1877), F. J. F. Meyen (1804-1840), Franz 

 Unger (1800-1870), and Carl von NageH (1807-1891). The multipUca- 

 tion of cells by division was observed by several investigators between 

 1830 and 1840, von Mohl being the first to describe the process in some 

 detail. 



As Sachs has pointed out, Schleiden's strange theory of cell-formation 

 could hardly have been developed if the true relation of cell-division to 

 what various workers had called free cell-formation had been recognized 

 earlier. Von Mohl (1835, 1844) maintained that there are two modes of 

 cell-formation: by division and by the formation of cells within cells; 

 but considerable uncertainty remained regarding the behavior of the 

 vacuolated protoplast ("primordial utricle") during these processes. 

 At the same time Nageli (1844) produced an exhaustive treatise on the 

 nucleus, cell-formation, and growth. In algse and the microsporocytes 

 of angiosperms he clearly showed that cells multiply by division, and 

 Schleiden was forced to admit that this might be "a second kind of cell- 

 formation." The continuation of Nageli's researches in 1846 completely 

 overthrew Schleiden's conception of free cell-formation, establishing the 

 significant fact that practically all vegetative cell-formation is by cell- 

 division. Many similar observations had been made by Unger and 

 von Mohl, but Nageli elaborated a broad theory which took into account 



