40 



AN INTRODUCTION TO MODERN GENETICS 



the chromosomes appear again they are still in contact with the 

 nucleoli. 



All the parts of the chromosomes which remain deeply stained in 

 the interphase are considered to be made of a material known as 

 heterochromatin, as opposed to the rest of the chromosomes which are 

 made of euchromatin. It is doubtful, however, how far all heterochro- 

 matin is really the same. The genetic properties of heterochromatin are 

 only known in a few cases. In Drosophila the "inert" regions of the 

 X and Y chromosomes, and the smaller inert regions near the centro- 



Fig. 5. Semi-diagrammatic Drawings of Vesicular Nuclei.— A in sperma- 

 togonium of grasshopper Aularches, note the more condensed condition of the X; 

 6 in spermatogonium of Phrynotettix, with a less condensed X. 



(A after White; 6 after Wenrich.) 



meres of the Ilnd and Ilird chromosomes, seem to contain no heredi- 

 tary factors; they are heterochromatic to the extent that they show 

 differential staining in mitosis^ and the permanent prophase of the 

 salivary glands nuclei (p. 98), but they are not all concerned in the 

 production of prochromosomes or nucleoli.^ The centromeres them- 

 selves are not considered to be made of heterochromatin: they are some- 

 what similar in staining properties to the centrosomes. 



In most resting nuclei we cannot see even the feeble traces of per- 

 sisting chromosomes which have been described above. The evidence 

 that they do persist is of a different kind and is overwhelming. Indivi- 

 dual chromosomes can often be recognized by their shape, size, and 

 peculiarities such as the distribution of constrictions, trabants, etc. 

 Cells which are derived from a single ancestral cell by mitotic division 

 are usually characterized not only by possessing the same number of 

 chromosomes, which might be merely a function of the amount of 



^ Rev. Heitz 1935. - Cf. Kaufman 1938. 



