4 THE CHRO^NIOSOMKS 



certain, since in some cases they become visible at the 

 beginning of one mitosis in the same position as they 

 occupied at the end of the ])receding division. ^^ 

 Also, in some cases special portions of chromosomes 

 persist in a condensed state throughout the resting 

 stage (proch romosomes ) . ^ * 



It is now becoming increasingly clear that no idea 

 of the structure of tlie resting nucleus can be o])tained 

 from studying fixed j)reparations. The usual text- 

 book figure of a ' network of linen tlireads with 

 granules of chromatin at the ])oints of intersection ' 

 is meaningless save as a descri])tion of a gross artefact 

 which bears only the most remote relation to the 

 living structure. We must resign ourselves to the 

 fiict that the resting nucleus and its chromosomes, 

 due probably to their high water content, are unfix- 

 able. It should be emphasized that this only a])])lies 

 to resting nuclei — thei'c is every reason to believe 

 that fixed preparations of nu( lei in mitosis present a 

 very accurate ])icture of what is taking place in the 

 living cell (see next cha])ter). 



Most cells in the body of an adult human being 

 hav^e undergoiK^ about 50 mitoses since th(* feitiUzed 

 egg (i.e. an adult man consists of about lO''* cells, 

 making allowance for the erythrocytes, s])erms and 

 other cells which are constantly being d(\stroye(l and 

 replaced). In the case of insects the adult cells have 

 undergone only about 20-30 divisions, and in the 

 case of the Nematoda and Rotifera still fewer. Be- 

 tween each of the^ divisions a resting stage has 

 intervened. These resting stages are, however, of 

 very uneven duration ; on the one hand two divisions 

 may follow on one another without being se})arate(l 

 by any resting stage at all (the end of one division 

 passing directly into the beginning of the next) — or on 

 the other hand the resting stage may last for years as 

 in many adult tissues of vertebrates. Many adult 

 cells may be said to have entered a i)ermanent resting 

 stage, since they will never divide again. In most 



