THE INTERPHASE NUCLEUS 



Walker and Tozer,^^ Ludford,^^ and Gates. ^^ ®® One of the main 

 questions has been the distinction between the true nucleolus or 

 plasmosome, and the residual 'heteropycnotic' chromatin of what has 

 been termed the chromosome-nucleolus. The two are usually closely 

 associated. The difference in staining reactions of these two nuclear 

 components was recognized as early as 1883 by Ogata. ^^ For those who 

 accept its validity, the Feulgen reaction clearly distinguishes between 

 chromosomal material and the main body of the nucleolus, which in 

 the opinion of most authors is Feulgen-negative. Wermel,^^ one of the 

 earliest users of the test, found this negative reaction to be particularly 

 distinct in mammalian neurones in which there was little residual 

 chromatin in the nucleus. In a series of insect oocyte nuclei Bauer®' 

 found but two instances of Feulgen-positive nucleoli, in which it may 

 be presumed that heterochromatic material was incorporated. In the 

 nucleolus of the mammalian liver cell (Davidson®*) there is an outer 

 zone of this Feulgen-positive material. 



Composition of true nucleolus 



The nucleolus proper is composed mainly of ribonucleoproteins. 

 Treatment with ribonuclease has been shown to decrease both the 

 ultraviolet absorption of this body (Davidson and Waymouth®^) and 

 also its affinity for basic dyes (Brachet,^* Gersch and Bodian,®* 

 Davidson et alii^"^). It is now usual to regard the basophilia of the 

 nucleolus as its most characteristic feature, though in the older litera- 

 ture, an affinity for acidic dyes is often described (Ludford,^' 

 Wilson®^). Ribonucleic acid is basophilic, but the basic proteins to 

 which it is bound within the nucleolus (Caspersson®^) can take up 

 acidic dyes at low />H's (Hyden'^^). Both these reactions are as yet 

 insufficiently understood. In developing oocytes, the nucleolus may 

 exhibit regions of different affinity [Limnea, Ludford^^; Asterias, 

 Gates®°) ; this condition is related to active changes within the nu- 

 cleolus, at the time when yolk is being deposited within the cytoplasm 

 of the developing oocyte. In Limnea, acidophilic material is then ex- 

 truded by the nucleolus through the nuclear membrane into the 

 cytoplasm; numerous nucleolar buds are formed at the same time 

 (Figure 10). Again, Hogben'^^ describes how the yolk granules of the 

 cockroach oocyte may be traced back to material from nucleolar 

 vacuoles. A remarkable example of another kind of secretory activity 

 which apparently originates in the nucleolus is provided by the slime 

 glands of the skin of Myxine (Schreiner'^^) which produce immense 

 quantities of material in all.* According to this author, fuchsinophilic 



* The statement in Parker and Haswell's Text-book of ^oology that 'two specimens of 

 Myxine thrown into a bucket of water are capable of gelatinizing the whole with their 

 secretion' is well known. 



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