68 Nature of the Genetic Material 



2. Another interpretation is derived from Ascaris, in \\hich an 

 important and remarkable difference between germinal and somatic 

 cells does indeed exist. Nematodes are cell-constant animals (Gold- 

 schmidt. 1903-1908; see Martini, 1924). This means that gro\\th and 

 differentiation take place largely without cell di\ision. Thus only 

 about eight di%isions of one cell are needed to produce all central 

 ganghon cells (162) of the animal ( Goldschmidt 190S). (The details 

 are a little more comphcated and variable than this brief statement 

 indicates, but it is essentially correct. See the most recent contribution 

 to the subject by Wessing. 195-3, who also found 162 ganghon cells in a 

 completely different nematode. ) The sex cells, however, divide indef- 

 initely. Thus we could conclude that heterochromatin is needed for 

 continuous cell di\"ision. Actually this conclusion already had been 

 reached from other dehberations (Darhngton), and it was even 

 pointed out that accumulation of heterochromatin beyond the normal 

 may be the cause of mahgnancy. Let us assume that this explanation 

 of diminution ^^ Goldschmidt and Lin, 1947) is correct for Ascaris. 

 \Miy, then, does diminution take place in Sciara and the cecidomyids? 

 Neither is cell-constant in the sense of the nematodes, though some 

 organs hke sahvar>- glands, epidermis, and oenocNtes seem to be cell- 

 constant in Diptera. -\nd, of course, the problem comes up again, why 

 only the few species show the phenomenon. It should be added that 

 Darlington and Thomas' (1941) obser\ation of extra di\'isions in the 

 pollen in the presence of extraheterochromatin is generally in harmony 

 with what Ascaris seems to teach. 



3. There is a third possibiHt\% almost the opposite of the second. 

 One of the differences bet^veen sexual and somatic cells in the animals 

 under consideration is this (again pointing to White's aberrant claim 

 for the gall midges ) : the germinal cells are early separated and then 

 do not di\-ide for a long time. The formation of a gonad \Wth in- 

 tensively di\iding sex cells occurs only late in development after all 

 the rest of the body is well differentiated. From this we could con- 

 clude that heterochromatin in bulk is a hindrance to quick division 

 and is therefore removed from the quickly dividing cells. We could 

 bring this into agreement with, the facts of cell constancy. Both 

 nematodes and insects have a great tendency to endomitosis, meaning 

 a duphcation of the chromosomes within the nucleus up to a high 

 pohploidy. In cell-constant animals and organs of other\Wse not cell- 

 constant animals this endomitosis seems to play a considerable role 

 in differentiation. Thus we might conclude that heterochromatin is 

 also a hindrance to endomitosis. This third h>'pothesis recognizes, like 



