Genie Control of Development 299 



sis of the situation, which shows that completely different problems 

 are involved in endomitosis and genie diversification ) by the fact that 

 the total content of DNA in endomitotic nuclei increases with the num- 

 ber of divisions in potencies of two. (See above, fig. 2; also Kurnick 

 and Herskowitz, 1952.) 



Schultz now argues that if intranuclear differentiation takes place 

 with endomitosis, it might also occur otherwise (though he himself 

 advises caution ) . It appears to him that observed differences of hetero- 

 chromatic parts of the chromosomes may be a case in question. He 

 points to the phenomenon of the variegated position effect in hetero- 

 chromatic breaks. Since the grade of variegation increases in differ- 

 ent tissues with the time of development of the organ, Schultz thinks 

 that a progressive change of the functions of the heterochromatic 

 regions follows. If it is assumed that the heterochromatic regions have 

 a function in the general metabolism of the chromosome, the change 

 of activity of the loci, transposed to heterochromatic neighborhood, 

 could be understood as a transformation in such a way that such loci 

 are able to carry out only the generalized functions of heterochroma- 

 tin and no longer their own specific activities. If now these variegated 

 rearrangements are regarded as indicative of processes going on in 

 normal development, "the possibility of nuclear differentiation in his- 

 togenesis appears." In embryogenesis only the specific genes required 

 for function in a particular type of cell are in an active state. The 

 others are in the heterochromatic state, and carry only the general 

 functions of such regions. This heterochromatization might be irre- 

 versible or reversible. Thus far Schultz argues. 



Our former discussions of all the facts concerned tend to show 

 that these conclusions cannot be accepted. The variegated position 

 effect probably demands a completely different interpretation, one 

 based upon developmental rather than genie features. Apart from 

 this, Schultz' theory tries to explain differentiation by intranuclear 

 changes, requiring so-called heterochromatization of "genes" with 

 change of their function from a special to a general one. I wonder 

 whether the order of events has not been turned around, just as it 

 was in polyteny. (See the discussion of scale formation, III 5 A a.) 

 Assuming for argument's sake that it had been shown that during cell 

 differentiation some chromosomal parts had been heterochromatinized 

 (though there is hardly a more confused subject in modem cytogenet- 

 ics than the claims for shifting heterochromatization of euchromatic 

 chromosomal regions; a perusal of Schultz' review, 1941, of this sub- 

 ject, as well as our former discussion, will bear out this statement). 



