THE STUDY OF STRUCTURE. 357 



The cell-doctrine has been already stated; in 

 its morphological aspect it emphasises the funda- 

 mental unity of minute structure in all living crea- 

 tures. The simplest organisms are single cells. All 

 other organisms are built up of many cells or modi- 

 fications of cells. Among themselves they show di- 

 vision of labour which is expressed in the great va- 

 riety of form and structural detail. From the fertil- 

 ised ovum onwards, the formation and growth of the 

 body is due to cell-division. This occurs in various 

 fashions, but especially in one complex (indirect or 

 karyokinetic) fashion which shows a fundamental 

 similarity throughout the entire series. 



Corroborations of the analysis into cells were 

 rapidly forthcoming. As early as 1824, Prevost and 

 Dumas had studied the cleavage of the fertilised 

 ovum, and it may be noted that some stages of this 

 can be seen with the naked eye in the relatively large 

 egg of the frog, which measures about one-tenth inch 

 in diameter. Similarly, Martin Barry (1838- 

 41), Reichert (1840), Henle (1841), Kolliker 

 (1843^6), and Eemak (1841-52) showed how the 

 cells of the embryo arise from the division of the 

 fertilised egg cell. 



Moreover, Goodsir in 1845, Virchow in 1858, 

 proved that in all cases, pathological as well as nor- 

 mal, cells arise from pre-existing cells, that omnis 

 cellula e cellula is a general fact of histology. 



There was a strong tendency, however, to attach 

 too much importance to the cell wall, and too little 

 to the contained cell substance. The all-important 

 protoplasm was not adequately appreciated. 



In 1835, Dujardin described the " sarcode " of 

 Protozoa, and other animal cells ; in 1839, Purkinje 

 compared the substance of the animal embryo (which 



