CHAPTER XXI 
THE BEARERS OF THE HERITAGE 
AN ACCOUNT OF THE CELLULAR BASIS OF HEREDITY? 
MICHAEL F. GUYER 
> Structure of the cell.—Before we can understand certain necessary 
details of the physical mechanism of inheritance we must inquire a 
little further into the finer structure of the cell and into the nature 
of cell-division. A typical cell, as it would appear after treatment 
with various stains which bring out the different parts more dis- 
tinctly, is shown in Fig. 43. Typical, not that any particular kind of 
living cell resembles it very closely in appearance, but because it shows 
in a diagrammatic way the essential parts of a cell. In the diagram, 
there are two well-marked regions: a central mucleus and a peripheral 
cell-body or cytoplasm. Other structures are pictured, but only a few 
of them need command our attention at present. At one side of the 
nucleus one observes a small dot or granule surrounded by a denser 
area of cytoplasm. This body is called the centrosome. The nucleus 
in this instance is bounded by a well-marked nuclear membrane and 
within it are several substances. What appear to be threads of a 
faintly staining material, the /inin, traverse it in every direction and 
form an apparent network. The parts on which we wish particularly 
to rivet our attention are the densely stained substances scattered along 
or imbedded in the strands of this network in irregular granules and 
patches. . This substance is called chromatin. It takes its name from 
the fact that it shows great affinity for certain stains and becomes 
intensely colored by them. This deeply colored portion of the cell, 
the chromatin, is by most biologists regarded as of great importance 
from the standpoint of heredity. One or more larger masses of 
chromatin or chromatin-like material, known as chromatin nucleoli, 
are often present, and not infrequently a small spheroidal body, 
differing in its staining reactions from the chromatin-nucleolus and 
sometimes called the true nucleolus, exists. 
Cell-division.—In the simplest type of cell-division the nucleus 
first constricts in the middle, and finally the two halves separate. 
From M. F. Guyer, Being Well Born (copyright 1916). Used by special 
permission of the publishers, The Bobbs-Merrill Company. 
299 
