CIL\PTER 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 t\T3ical cell, as it would appear after treatment 

 with various stains which bring out the different ' parts more dis- 

 tinctly, is shown in Fig. 43. T\'pical, 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 nucleus 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 linin, 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 

 chrom.atin 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 jM. F. Guyer, Being Well Born (copyright 19 16). Used by special 

 permission of the publishers, The Bobbs-Merrill Company. 



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