THE INTERPHASE NUCLEUS 



of the nucleolar material, but even when the nucleoli are first recog- 

 nizable, their number is variable, Lewis states that this is true both of 

 mother and daughter nuclei and of the daughter pairs. According to 

 Fell and Hughes nucleoli seem first to form where chromosomes are 

 most densely clumped together in the early telophase nucleus. It 

 may be that a number of chromosomes are concerned in nucleolar 

 formation, and that their several individual contributions may be 



Figure 12 Diagram of chromosome pair in the half-sized ovarian 

 eggs of Amphibia (compare Figure 35e). Paired granules are shown 

 imbedded in 'plastic cylinder'. Letters indicate various lateral loops, 

 formed as outgrowths of the granules. Numbers indicate successive 

 stages in formation of a nucleolus at a definite locus, and its passage 

 through the nuclear membrane. From Duryee** {By courtesy. 

 University of Pennsylvania Press) . 



indistinguishably fused from the very first. In the nuclei of malignant 

 cells with a diploid number of polytene chromosomes, the number of 

 nucleoli is increased (Biesele et alii^^^) . The duplicated chromonemata 

 must therefore each carry separate nucleolar organizers. 



HETEROPYCNOSIS 



It was observed by Henking^^^ that a 'peculiar chromatin element' 

 could be seen in both resting and dividing stages of the spermatocytes 

 of the Hemipteran Pyrrhocoris. During the anaphase II of meiosis, this 

 element passed undivided to one pole, and was thus to be found in half 

 the resulting spermatids. Similar observations in the following years 

 were made on other insects ; this body was regarded by most authors as 

 chromosomal in nature, although Henking had regarded it as a nu- 

 cleolus. The suggestion that their distribution to half the spermatids 

 was related to sex determination was due to MgClung^"^; the term 

 'heterochromosome' for such bodies was first used by Montgomery^"^ 

 in 1904. In the spermatogonia! mitoses, the heterochromosomes do not 

 lose their chromatin at telophase at the same rate as do the other 

 chromosomes; they are visible within the interphase nucleus. 



43 



