42 AGE, GROWTH, AND DEATH 



ents we may call the sap (hyaloplasma), the fluid 

 material which fills out the meshes of the network. 

 Later on we shall have occasion to study somewhat 

 more carefully the principal variations which nuclei 

 of different kinds may present to us, and we shall 

 learn from such study that we may derive some 

 further insight into the rapidity of development and 

 the nature of the changes which result in old age. 

 While the picture is upon the screen, I wish to call 

 your attention to the other figures, which illustrate 

 the process of cell multiplication. As you regard 

 them you will notice in the succession of illustrations 

 that the nucleus has greatly changed its appearance. 

 The substance of the nucleus has gathered into sepa- 

 rate peculiarly elongated granules, each of which is 

 termed a chromosome. The chromosomes are very 

 conspicuous under the microscope, because they ab- 

 sorb artificial stains of many sorts with great avidity 

 and stand out therefore conspicuously coloured in 

 our microscopic preparations. They are much more 

 conspicuous than is the substance of the resting 

 nucleus. The fact that we can readily distinguish 

 the dividing from the resting nucleus under the 

 microscope, compare Fig. 72, we shall take advan- 

 tage of later on, for it offers us a means of investigat- 

 ing the rate of growth in various parts of the body. 

 I should like, therefore, to emphasise the fact at the 

 present time sufficiently to be sure that it will remain 



render it probable that there is in the network only one constituent of which 

 the chromatine is a functional modification, varying in extent in accordance 

 with the alternating phases of cell-life. 



