8O GENERAL PRINCIPLES OF ZOOLOGY. 



nucleoli, which is filled by a white-of-egg-like fluid, the 

 nuclear sap. 



Nuclein and Paranuclein. Here now we encounter a 

 mooted question : is there one or are there several kinds 

 of nuclear substance ? We shall here distinguish two kinds, 

 the nuclcin or the chromatin, and the paranuclein or the 

 acJiromatin. The difference becomes apparent by the 

 parts which they take in the process of cell division (de- 

 scribed later) and by the following reaction to dyestuffs 

 (carmine, haematoxylin, aniline solutions). By correct 

 use of the latter only the nuclein is stained (hence called 

 chromatin), while the paranuclein remains entirely un- 

 stained. Each of these two substances may appear in the 

 form of a network as well as of nucleoli, so that we must 

 distinguish chromatic and achromatic networks and nucleoli. 

 Most frequent is an achromatic network with chromatic 

 granules scattered in it (Fig. 19, g}. 



Significance of the Cell Nucleus. For a long time 

 the functional significance of the nucleus in the cell was 

 shrouded in complete darkness, so that it began to be 

 regarded in comparison with the protoplasm as a thing of 

 little importance. The evidence that the nucleus plays 

 the most prominent role in all fertilization processes, and 

 the numerous researches on this point, have more and more 

 confirmed the view that the nucleus determines the char- 

 acter of the cell ; that the potentiality of the protoplasm is 

 influenced by the nucleus. If from the egg a definite kind 

 of animal develops, if a cell in the animal's body assumes 

 a definite histological character, we are, at the present time, 

 inclined to ascribe this to the nucleus. From this, then, it 

 follows farther that the nucleus is also the bearer of he- 

 redity ; for the transmission of the parental characteristics 

 to the children (a fact shown to us by our daily experience) 

 can only be accomplished through the sexual cells of the 

 parents, the egg and sperm cells. Again, since the char- 

 acter of the sexual cells is determined by the nucleus, the 

 transmission in its ultimate analysis is carried on by the 

 nucleus. Within the nucleus the chromatin is probably 



