24 PROTOPLASM AND THE CELL. 



composed of a soft, lifeless material closely surrounding the cell 

 substance.* 



As a second example we choose the growing point of a com- 

 mon water-plant (Ckara), Fig. 13. This structure is composed 

 of cells which are more or less angular in outline as a result of 

 mutual pressure, but show otherwise an unmistakable similarity 

 to the egg-cell just described. They differ mainly in the fact 

 that the protoplasm of the larger cells contains rounded cavities, 

 known as vacuoles, filled with sap (v) ; also in the chemical com- 

 position L f the cell-walls (here consisting of ' ' cellulose, ' ' a sub- 

 stance of rare occurrence among animals). 



Origin of Cells and Genesis of the Body. The body of every 

 higher plant or animal arises from a single germ-cell (" egg,' 

 "spore," etc.) more or less nearly similar to that of the star- 

 fish, described above, and originally forming a part of the parent 

 body. The germ-cell, therefore, in spite of endless variations in 

 detail, shows us the model after which all others are built ; for 

 it gives rise to all the cells of the body by a continued process 

 of segmentation as follows : 



The first step (Fig. 14) consists in the division of the egg 

 into two similar halves, which differ from the original cell only 

 in lacking membranes, both being surrounded by the membrane 

 of the original cell. Each of the halves divides into two, mak- 

 ing four in all ; these again into two, making eight, and so on 

 throughout the earlier part of the development. By this process 

 (known as the cleavage or segmentation of the egg) the germ- 

 cell gives rise successively to 2, 4, 8, 16, 32, 64, etc., de- 

 scendants, forming a primitive body composed of a mass of 

 nearly similar cells, out of which, by still further division and 

 growth, the fully-formed body of the future animal is to be 

 built up. These cells are only slightly modified, but differ in 

 most animals from the typical germ-cell in having at first no sur- 

 rounding membranes. The membrane of the original germ- 

 cell meanwhile disappears. 



* The word cell has been used in Chap. I and elsewhere to denote the 

 living matter within the membrane, the latter being considered a product of 

 the cell rather than an integral part of it. It is more usual to include the 

 membrane in a definition of the cell, and as a matter of convenience it is so 

 included here. 



