THE MITOTIC CYCLE 



telophase can now be regarded as characteristic of the species'. The 

 various types of relationship of chromosomes to nucleoli have been 

 summarized by Darlington^^; they imply that the number of nucleoli 

 in the early normal diploid telophase nucleus is constant. The greater 

 part of the literature on this particular topic refers to plant material ; 

 of the few corresponding papers on animal nucleoli, we may instance 

 those of Bearing ®° on Ambystoma tigrinum (Figure ii); Kaufman^^ on 

 the salivary gland nuclei o{ Drosophila melanogaster; and Chen^^ on the 

 primitive ciliate ^elleriella intermedia. In the nuclei of heteroploid 

 Amphibia, the nucleoli have been found to be related to the degree of 

 ploidy. Fankhauser and Humphrey ^^ have used the number present 

 as an index of heteroploidy, while Beatty^* finds that the sum of the 

 areas of the nucleoli in a nucleus is proportional to the number of chro- 

 mosomes. Kawaguchi^^ has shown that in nuclei of polyploid silk- 

 worms, there is an equality between the number of nucleoli and the 



& 



Figure 1 1 The nucleolus bearing chromosome in Ambystoma 

 tigrinum in larval epithelial nucleoli; from very early to late 

 prophase. X 2,000. From Dearing^" (By courtesy, jf. Morph.). 



number of the 'Z' chromosomes, which are one of the two types of sex 

 chromosome. Duryee^^ has shown that the cloud of nucleoli which are 

 formed during the 'lamp-brush' stage in the oocyte nucleus of the frog 

 all arise from nucleolar organizers in particular chromosome pairs 

 (Figure 12). 



In Mammalia, the evidence for relationships between nucleoli and 

 chromosomes is somewhat conflicting. Schultz and Lawrence ^^ find 

 that in human spermatocytes there are two nucleoli, one borne on the 

 sex chromosome, the other on an autosome to which these a,pthors have 

 devoted particular study. On the other hand, Andreassi^' finds that the 

 nuclei of the human foetal liver contains three or more nucleoli, which 

 tend subsequently to fuse together. In the rat, the maximum number of 

 nucleoli in diploid nuclei is six (Biesele^^), and in the mouse is four 

 (Biesele^^). In studies on fibroblastic cells of these animals in tissue 

 culture, neither Lewis^°° nor Fell and Hughes^'^^ were able to observe 

 any constancy in number or arrangement of the nucleoli in telophase 

 nuclei. In the early interphase nucleus, there is much re-arrangement 



42 



