CENTROSOME. 25 



ulated and its life destroyed. Acids and strong alkalies have the 

 power of destroying the movement altogether, while chloroform 

 inhibits it temporarily. This property of responding to a stimulus 

 is known as irritability, and the fact that a stimulus applied to one 

 part of a mass of protoplasm will produce results in other and 

 distant parts demonstrates the presence of conductivity. 



Nutrition. Another property possessed by living protoplasm 

 is that of nutrition ; by which is meant the power to absorb mate- 

 rial, to convert it into protoplasm, and to get rid of such waste 

 products as have served their purpose or are formed as a result of 

 the activity of the protoplasm. That portion of the process which 

 is concerned in the building up of the protoplasm is assimilation 

 or anabolism, while that concerned with its breaking down or 

 destruction is disassimilation or katabolism. 



A fourth property of protoplasm is that of reproduction, which 

 will be treated of under the heading Division of Cells. 



Nucleus. Embedded in the protoplasm is a vesicle of various 

 shapes spherical, oval, or irregular which is to be regarded as 

 of great importance, especially in the process of cell-subdivision 

 by which new cells are formed and growth thus brought about. 

 It consists of an external enveloping membrane, the nuclear 

 membrane, enclosing the chromoplasm or intranuclear network, 

 a material resembling spongioplasm, and in the interstices of this 

 is the nuclear matrix. In addition to these there are nucleoli, 

 some of which are thickenings of the network like the knots in 

 the spongioplasm, and are called pseudonucleolij while others are 

 free, the latter being the nucleoli proper, or the true nucleoli. A 

 single true nucleolus is usually found, although this is not always 

 the case. 



Chromatin and Achromatin. When cells are stained with hema- 

 toxylin the nuclear membrane, the chromoplasm, and the nucleoli 

 take up the staining-fluid readily, while the nuclear matrix does 

 not ; hence the former are said to be made up of chromatin, or to 

 be chromatic ; while the latter is achromatin, or is said to be achro- 

 matic. Other dyes, such as safranin, methyl-green, and carmine, 

 produce the same effect: Chromatin is but another name for 

 nuclein, which is the principal constituent of the nucleus. It is 

 closely allied to the proteids, but is characterized by containing 

 a considerable percentage of phosphorus ; some analyses give as 

 much as 8 per cent. Nuclein is a compound of nucleic acid with 

 proteids, and it is to the affinity of this acid for the coloring- 

 matter that the staining of chromatin is due. It is more correct to 

 speak of nucleins rather than of a single substance, as the compo- 

 sition of nuclein is not always the same. For a further discussion 

 of this subject the reader is referred to the chapter dealing with 

 Proteids. 



Centrosome. As already stated, this is probably to be regarded 



