STRUCTURE OF CELLS. 39 



This doctrine was corroborated in many ways. Numerous investi- 

 gators, Prevost and Dumas (1824), Martin Barry (1838-41), Reichert 

 (1840), Henle (1841), Kolliker {1843-6), and Remak (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 normal, 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 183s, Dujardin described the "sarcode" of Protozoa, and other 

 animal cells ; in 1839, Purkinje compared the substance of the animal 

 embryo with the "cambium " of plant-cells ; in 1846, Von Mohl emphas- 

 ised the importance of the "protoplasm" in vegetable cells; Ecker 

 (1849) compared the contractile substance of muscles with the living 

 matter of amoebse ; Bonders also referred the contractility from the wall 

 to the contents ; Cohn suspected that the " sarcode " of animals and the 

 "protoplasm" of plants must be "in the highest degree analogous 

 substances;" and finally. Max Schultze (l86i), accepted the growing 

 belief that plants and animals were made of very similar living matter, 

 and defined the cell as a unit mass of nucleated protoplasm. Since then 

 biologists have concentrated their attention on the living matter which 

 constitutes and gives form to the cell. 



Forms of Cells. — The typical and primitive form of cell is 

 a sphere, — a shape naturally assumed by a complex coherent 

 substance situated in a medium different from itself. Most 

 egg-cells and many Protozoa retain this primitive form, 

 but the internal and external conditions of life (such as 

 nutrition and pressures), often evolve other shapes, — oval, 

 rectangular, flattened, thread-like. A cell has often two 

 distinct poles, as we noticed in connection with epithelium, 

 but some, like amcebse and colourless blood-cells, are con- 

 tinually changeful, while others give off radiating processes 

 on all sides. 



Structure of Cells. — Cells consist (a) of the living sub- 

 stance or protoplasm, along with which we may include 

 nutritive material in process of being incorporated, waste- 

 products which result from the vital activity, and by-products 

 formed in the course of the cell's life ; \b) of a specialised 

 kernel or nucleus, with a complex structure, and important 

 but hardly definable functions in the life of the cell; (c) of a 

 cell-wall or marginal sheath, which occurs in very varied 

 form or may be entirely absent. 



(a) The Cell-Substance. — When a simple cell is examined 



