Genie and Non-genic Parts of the Chromosome 67 



and removed from the nucleus in prospective somatic cells. With 

 Boveri we can safely assume (as is experimentally proved in Ascaris 

 and clear from the morphological facts of the early separation of 

 germ-line material in the egg) that the kind of cytoplasm in which 

 the nuclei lie controls that nuclear process. It may be safely assumed 

 also that the absence or weakening of the centromere in the hetero- 

 chromatic chromosomes plays the decisive role in the mechanics of 

 diminution. In the comparable plant case (Darlington and Upcott, 

 1941; Darlington and Thomas, 1941) it can be stated that there is at 

 least a great similarity to diminution in animals, as far as the difference 

 between germinal and somatic cells is involved. Hence the function of 

 the heterochromatin must be involved in the differences between 

 somatic and germinal cells. We can visualize at present three major pos- 

 sibilities : 



1. Heterochromatin is a substance which is needed in the syn- 

 thetic p'jocesses of development and therefore reserved for the sex 

 cells. This view encounters serious difficulties. Both in Ascar'is and in 

 insects the few sex cells (two primary ones in Kscaris) are separated 

 early in development and start forming gonads only much later, while 

 all the features of development are concentrated in the somatic cells. 

 The idea under discussion could apply only to the processes taking 

 place in the egg in the next generation, that is, the growth and 

 organization of the oocyte, a time at which great visible changes do 

 occur in the nucleus and in its relations with the cytoplasm. We 

 could object that this would not apply to the sperm cells, but since 

 we know that one and the same primordial sex cell can develop into 

 an egg or a sperm and this actually happens in insects (Lymantria), 

 we should not expect differences between female and male cells in 

 this respect. If this idea were correct we should expect to find dimi- 

 nution in all animals with large organized eggs (predetermined eggs, 

 in the language of the experimental embryologist). Ascaris could 

 only partly belong to this group, but all insects and amphibia would. 

 It would be hard to understand why it is found only in Sciara (not 

 even in all species!), the cecidomyids, and the chironomid family, 

 though here White claims heterochromasy for the somatic cells. The 

 Dyticus case could be interpreted in this way without difficulty, and 

 it is always possible to account for the rarity of such events by saying 

 that the process actually occurs in all eggs or large eggs but is not 

 visible because the heterochromatin is finely divided. This does not 

 appear to be a very happy solution, and it does not fit the super- 

 numeraries of plants at all. 



