98 BIOLOGY: GENERAL AND MEDICAL 



cells of different kinds. It is usually enclosed in a hya- 

 line membrane, known as the nuclear membrane, and 

 consists of substance divisible into a part that is struc- 

 tureless and probably fluid, known as the nuclear juice, 

 or karyoplasm, and material that is for the most part 

 filamentous and known as the nuclear substance or 

 karyomitome. Staining reagents show that the struc- 

 tures comprising the nucleus are chemically different 

 from those of the cytoplasm, and a large part of the 

 science of histology has been the result of observations 

 made upon cells and tissues subjected to what is called 

 nuclear staining. 



The microchemic study of the nuclear structures has 

 not been very useful, except that it has shown them to be 

 dissimilar in composition. The nuclear membrane is said 

 to consist of amphipyrenin, the karyoplasm of paralinin, 

 and the karyomitome of two substances, one of which 

 does not react with the stains, and is called linin, the 

 other with a strong affinity for all nuclear stains, being 

 known as nuclein or chromatin. In some nuclei a small 

 distinct body or nucleolus occurs. It is composed of 

 pyrenin. 



Of these various structures the nuclein or chromatin 

 is of paramount importance, being composed of a number 

 of units, distinctly visible only at the time of cell multipli- 

 cation and known as chromosomes. Much will be said of 

 these bodies in connection with karyokinesis or nuclear 

 division and in considering the problems of heredity. 



The great majority of cells possess a single nucleus; 

 some of the unicellular organisms, as the protozoa are 

 provided with two, the larger and more distinct being 

 known as the macronucleus, the smaller and less con- 

 spicuous as the micronucleus or paranucleus. 



Some lowly organisms of considerable size consist of 

 masses of undifferentiated protoplasm containing many 

 nuclei. Such are known as plasmodia, and in some cases 

 as the myxomycetes, are known to be formed by the 

 coalescence of many cells. 



