THE STUDY OF ZOOLOGY 



15 



lacking, a cytoplasmic mass may contain numerous nuclei, all of 

 which are alike. Protozoa often show two kinds of nuclei in a 

 single cell. Thus in Paramecium (Fig. 2) there is a small 

 spherical micronucleus embedded in the side of the large kidney- 

 shaped macronucleus. In some species there are two or more 

 micronuclei. By microdissection it can be demonstrated that the 

 nucleus is bounded by a definite membrane. Some cells, such as 

 mammalian red blood corpuscles in the blood stream, lack a 



Fig. 1. — Cells. A, three cells from the outer layer of the skin of the sala- 

 mander one of which (d) is dividing, c, chromosomes; o, yolk globules; n, 

 nucleus; B, blood cell of the salamander, nucleated. C, human red blood cells, 

 non-nucleated. X 900. 



nucleus though each corpuscle is a nucleated cell at an earlier 

 stage of its development. Properly speaking, such a corpuscle 

 is only part of a cell and its life in the blood stream is of short 

 duration (about ten days in the case of man). 



The nucleus contains an important substance called chromatin 

 which, unless the cell is dividing, appears in fixed and stained 

 sections as flocculent masses of irregular outline. In the course 

 of cell division the chromatin is transformed into very distinct 

 bodies called chromosomes, each of which then divides in half, so 

 that both daughter cells receive the same number and kinds of 

 chromosomes. Chromosomes are important in connection with 



