66 Nature of the Genetic Material 



statement that pollen grains with an accumulation of supernumeraries 

 may undergo some extra divisions. 



An apparently completely different set of facts seems to me to 

 continue the line of variations thus far mentioned, though the dif- 

 ference between germ cells and somatic cells is not involved in the 

 same way. This is the remarkable unequal division discovered by 

 Giardina (1901) in the ovary of water beetles (Dyticus) (fig. 8). Here 

 the young egg cell undergoes four unequal divisions in which fifteen 

 nurse cells are separated from the oocyte. In the prophase of these 

 divisions the heterochromatin is assembled into a huge chromocenter 

 which occupies the major part of the nucleus. In metaphase this mass, 

 which does not assume the form of chromosomes, surrounds the 

 spindle in the form of a ring. This ring remains in the prospective 

 oocyte and is again incorporated into the nucleus. For a long time 

 these facts remained unique. Recently Bauer and students (see Bauer, 

 1952) found a very similar though not completely identical case in 

 Tipula (Diptera). He calls what I consider to be a chromocenter "a 

 chromosomal product," which is not very helpful. I suspect that many 

 other examples of this type will turn up in time. I described (Gold- 

 schmidt, 1950a) strange structures in the oocytes of a deep sea fish 

 which most probably belong to the same category, with some special 

 features pointing to origin from chromosome ends. 



c. Interpretations derived from the cytologicai facts 



The question now arises whether this first set of facts permits 

 any conclusions in regard to the function of the heterochromatin. 

 Some geneticists believe that heterochromatin is inert material and 

 that it is possible that heterochromatic chromosomes are functionless 

 and frequently lost in phylogeny, but the facts concerning diminution 

 of heterochromatin certainly do not agree with such a point of view. 

 The individual processes are so precise and constant where they are 

 found that they must have a definite meaning. Even the simple 

 cytologicai aspect militates against such a view. In figure 6 of Ascaris 

 oocytes, the heterochromatic chromocenter contains most of the 

 formed material of the nucleus; in Sciora the heterochromatic chro- 

 mosomes are unusually large; and in the cecidomyids they are five- 

 sixths of the chromosomal material. However, there is also a dia- 

 metrically opposed interpretation: White (1950) thinks that the 

 somatic chromosomes of the gall midges are the heterochromatic ones! 

 Now in all the other animal material, heterochromatin in the form of 

 chromosomes or chromosome parts is kept intact in the germ cells 



