THE CELL. 13 



The chromatic nuclear substance, as its name implies, has an 

 affinity for coloring-matter in the form of different stains. It is 

 usually in the form of a continuous network, but sometimes appears 

 in small granules, or particles. . It contains chromatin or nuclein. 



The nucleolus, if it appears at all, is found in the network of the 

 nucleus, as a rounded or irregularly shaped body. It contains para- 

 nuclein or pyrenin and has an especial affinity for color, and stains 

 more deeply than the network. The nucleoli are thought to be 

 passive bodies that hold in reserve different constituents which are 

 essential to the life of the nucleus. 



Sometimes the nucleus is enveloped in a membrane, called the 

 nuclear membrane, which marks it distinctly from the protoplasm. 

 This, however, as with the cell-membrane, is not universal and is not 

 classed as a general constituent of the nucleus. The sharpness of the 

 contour which distinguishes the nucleus in the midst of protoplasm 

 led many histologists firmly to believe that the nucleus always does 

 possess a membrane. The truth is between the two extreme opinions. 

 The nucleus can very readily exist without one. 



The nuclear membrane consists of an achromatic substance, 

 amphipyrenin. 



A portion of a cell deprived of its nucleus may live for a time, 

 but it evinces no activities or functions other than that of move- 

 ment. It neither absorbs food, nor grows, nor reproduces, but seems 

 gradually to dwindle away and die. From this it is believed that the 

 nucleus exercises some powers with regard to the building up, or con- 

 structive metamorphosis. 



Regarded chemically, the nucleus is composed principally of pro- 

 teid and a substance like proteid, which contains as much as 10 per 

 cent, of phosphorus. No doubt there are others, but even the most 

 delicate chemical reagents kill the constituents and so lessen the 

 opportunities for careful investigation. 



CENTROSOME. 



About twenty years ago, when nuclear cell-division was being 

 investigated, a small body other than the nucleus was noticed during 

 the division of the cell and was called by various names: polar 

 corpuscle, central corpuscle,' or centrosome. The last name seems to 

 be more generally used at the present time. 



The centrosome in its simplest form is a body of extreme min- 

 uteness, frequently not larger than a microsome, but which exerts an 

 active influence on the protoplasmic structure during cell-division. 



