TYPICAL CELLS. 



49 



lions of the cells. And so with other functions and the life of 

 the organism generally. 



The present chapter will deal chiefly with the form and struc- 

 tural relations of cells. 



A Typical Cell. The simplest way to understand cells and 

 their modifications is to study their development. The germ 

 (egg, spore, etc.) from which every higher plant or animal arises 

 is at first a single cell, called the germ-cell, which forms part of 

 the body of the parent and 

 is essentially similar to other 

 cells. Sooner or later the 

 germ-cell is detached from 

 the parent, and gives rise 

 by a process of multiplica- 

 tion and growth to a mass 

 of cells which constitutes a 

 new individual, like the 

 parent. The structure of 

 such a germ-cell is well 

 illustrated by the egg of a 

 star-fish (Fig! 20). The egg 

 of every animal, even in- 

 cluding man, shows the same 



essential structure : that of the star-fish shows it with especial 

 clearness, and this may be taken as typical, not only of all eggs, 

 spores, etc., but also of all cells. 



Form. Like most free or isolated cells, the egg is nearly 

 spherical. In the tissues, cells usually have angular contours as a 

 result of mutual pressure just as soap-bubbles, which are spheri- 

 cal when free, become polyhedral and bounded by plane surfaces 

 when a number of them are brought in contact. The sphere is 

 therefore regarded as representing the typical form of the cell ; 

 and from this all other forms are derived. 



Structure. In such an egg, as in any other typical cell, three 

 parts may be distinguished. These are (1) cell-substance, (2) 

 nucleus, (3) membrane (or cell-wall). 



In perfectly typical cells the cell- substance is simply a mass 

 of unmodified protoplasm. In the star-fish ovum it contains also 



scattered granules of lifeless proteid and fatty substances (here 



4 



FIG. 20. Slightly diagrammatic figure of the 

 egg or ovum of a star-fish, showing the struc- 

 ture of a typical cell; m, membrane; ji, nu- 

 cleus; p, protoplasm. 



