HISTORICAL SKETCH 9 



with Schwann, who was the one to formulate the Cell Theory in a com- 

 prehensive manner. Schwann announced the theory in concise form in 

 1838, and in 1839 published a very full account under the title "Mikro- 

 skopische Untersuchungen iiber die Uebereinstimmung in der Struktur und 

 dem Wachsthum der Thiere und Pflanzen." He says: "The elementary 

 parts of all tissues are formed of cells in an analogous, though very diver- 

 sified manner, so that it may be asserted that there is one universal prin- 

 ciple of development for the elementary parts of organisms, however different, 

 and that this principle is the formation of cells." And further: "The 

 development of the proposition that there exists one general principle 

 for the formation of all organic productions, and that this principle is 

 the formation of cells, as well as the conclusions which may be drawn from 

 this proposition, may be comprised under the term Cell Theory . . . " 

 " . . .all organized bodies are composed of essentially similar parts, 

 namely, of cells . . . " 



Elaboration of the Cell Theory. The Cell Theory at once became 

 established as one of the main foundation stones of biological research, 

 but it underwent considerable modification as investigations proceeded. 

 The main thesis, that the body is composed of cells and their products, 

 remained, but other ideas associated with this in the minds of Schleiden 

 and Schwann, particularly that concerning free cell-formation, were 

 superseded. Soon after the formulation of the Cell Theory its elabora- 

 tion was begun by Unger, von Mohl, and Nageli, who based their con- 

 clusions on observations of a very high order. Franz Unger (1800-1870), 

 in two works appearing in 1844 on vegetable growing points and the 

 growth of internodes, argued for the origin of cells by division. Von 

 Mohl, in two treatises (1835, 1844), maintained that there are two meth- 

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

 cells. He thought the "primordial utricle" (protoplast) must be ab- 

 sorbed to make way for the two new ones, or, less probably, the old one 

 must divide into two. Like Schleiden, he thought the nucleus must be 

 incorporated in the cell wall, but later (1846) concluded that it lies in 

 the primordial utricle. It was in his paper of 1846 that von Mohl in- 

 troduced the term "protoplasm" in its present sense. 



Carl von Naj^li (1807-1891) in 1844 produced an exhaustive treatise 

 on the nucleus, cell-formation, and growth. In algae and the micro- 

 sporocytes of angiosperms he clearly showed that cells multiply b}' 

 division, and Schleiden was forced to admit that this might be "a second 

 kind of cell-formation." The continuation of Naegli's researches in 

 1846 completely overthrew Schleiden's conception of free cell-formation, 

 establishing the significant fact that 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 

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



