ELEMENTARY VITAL PHENOMENA 



189 



protoplasm belonging to them remains in one mass ; the final result 

 is a very large but multinucleate cell. 



Every cell exhibits, if not continually, at least at a certain time 

 of its life, phenomena of growth ; the mass of its living substance 

 increases. This can occur only by taking in material from the 

 outside, or, in other words, by metabolism ; and the conception of 

 growth can be rendered precise by bearing in mind that in meta- 



Fio. 68. 7. Formation of eggs in the sea-urchin. A, Piece of a young ovary with the germinal 

 epithelium within ; B, piece of an older ovary, in which the cells of the germinal epithelium 

 are developing into eggs which are being constricted off. (After Ludwig.) //. Egg-tubes of 

 the ovary of an insect. In the tubes lie eggs in different stages of formation. (After 

 Hatschek.) 



bolism more living substance is built up than is broken down. But, 

 as has been seen, the size of every cell is limited and does not 

 surpass a certain measure. Particularly the size of every definite 

 cell-form has a limit assigned for that particular form, which varies 

 little. Hence, if the quantity of the living substance increases 

 further by growth, this must lead to a " growth beyond the measure 

 of the individual," the cell-mass must divide, i.e., it reproduces. 

 The cell, therefore, multiplies by division ; and every one of the 



