74 Nature of the Genetic Material 



weak bands. But the Y-chromosome contains a small pairing section 

 (in the bobbed region) which, if visible, does not affect our ideas 

 about the chromocenter. The chromocenter usually appears as an 

 unorganized, granular, or even degenerating mass, which is easily 

 broken in preparation, sometimes even absent. Now it must be kept in 

 mind that a salivary gland cell is highly specialized and may not need 

 the heterochromatic parts which form a chromocenter in interphase 

 nuclei of actively dividing cells where it is very conspicuous and 

 deeply stained. As block heterochromatin is certainly involved some- 

 how in cell division, it might even be expected that salivary nuclei do 

 not show much of it. It may have been destroyed during the growth 

 phase of these cells. The condition in the salivary gland cells might 

 better be described in a negative way, as the absence of block hetero- 

 chromatin from the chromosomes. This would also agree with the 

 fact that in Chironomus species salivary nuclei without chromocenters 

 are found. Nevertheless, in certain species a large part of the hetero- 

 chromatin exists as block or chromocentral heterochromatin which 

 may be removed from the chromosomes in interphase and assembled 

 into huge Feulgen positive masses filling a major part of the nucleus 

 in such an extreme case as Ascaris (fig. 6), 



The next problem is to inquire into possible functions of this 

 material. A considerable number of facts are known from which con- 

 clusions may be drawn. In D. melanogaster the Y has only a very 

 small pairing segment, which may be considered as euchromatic. All 

 the rest is heterochromatic and clearly does not contain sex-deter- 

 mining units (the general meaning of which will be discussed in a 

 chapter on sex). Males without or with only a part of the Y are normal 

 (though sterile) and so are females with one or more extra Y. The 

 sterility of males without Y has been attributed to the presence of 

 fertility genes in the Y, which have even been "counted" by some 

 authors (Neuhaus, 1939). Actually, the sterility action is confined to 

 a certain part of the Y (Stern, 1929Zj), but we know very little what 

 this sterility means. Shen (1932) found completely normal spermio- 

 genesis in males without a Y; but when the spermatozoa reach a 

 certain part of the vas deferens they are immobilized (see also Stern 

 and Hadorn, 1938). Now we know, at least for Lepidoptera, that the 

 motility of the spermatozoa is dependent upon secretions in the 

 female bursa copulatrix (Klatt, 1919). Thus, we may suppose that 

 the immotility of the sperm in the absence of the Y-chromosome may 

 be caused by some small chemical change in the secretion of the vas 

 deferens. From this we may conclude that the absence of certain parts 



