50 



THE CELL. 



called deutoplasiri), which serve as a store of food for the de- 

 veloping embryo. 



The nucleus is a rounded body suspended in the cell-sub- 

 stance ; it is distinguishable from the latter by its higher refrac- 

 tive power, and by the intense color it assumes when treated with 

 staining fluids. It is surrounded by a very thin membrane, and 

 consists internally of a clear fluid (achromatiii), through which 

 extends an irregular network of fibres (chromatin). It is espe- 

 cially these fibres which are stained by dyes. In the meshes of 





FIG. 21. Cleavage or segmentation of an ovum, showing successive division of the germ- 

 cell (a ) into two (b\ four (c), and eight (d). Later stages are shown at e and /. The first 

 four figures are diagrammatic; e and /are after Hatschek's figures of the development 

 of a very simple vertebrate (Amphioxus). 



the network is suspended a second rounded body known as the 

 nucleolus, which stains even more deeply than the network itself. 



The membrane or wall of the cell forms a rather thick sac, 

 composed of a soft, lifeless material closely surrounding the cell- 

 substance.* 



Cells in the Body. Typical cells, closely similar to that just 

 described, are found here and there throughout the vegetal and 

 animal kingdoms in the most unlike organisms. Such cells show 



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. 



