THE MITOTIC CYCLE 



both of these will vary according to the proportions and also to the 

 degree of orientation of each component. Caspersson^*' has found 

 that the nucleic acids in the bands of the larval salivary chromosomes 

 of Drosophila are not highly orientated, although sufficiently orientated 

 to produce an overall negative birefringence (Schmidt^*^). In the 

 interband regions which consist mainly of protein, the orientation 

 increases on stretching to give a positive birefringence (Schmitt^*^). 

 Such studies should be extended to the 'lamp-brush' chromosomes of 

 the vertebrate oocyte, which in virtue of their looser structure might 



Figure 34 Diagram of nuclear growth stages during the later development 

 of frog eggs. From Duryee.^*^ a & b Stages i and 2 : Egg < 02 mm. in 

 diameter. Chromosomes barely visible in life, c Stage 3: Egg o-2-o*5 

 mm. in diameter. Large irregular nucleoli just beneath nuclear mem- 

 brane. Chromosomes forming lateral loops, d Stage 4 : Eggs 0-5-0-75 mm. 

 in diameter. Chromosomes reach maximum length, e Stage 5: Eggs 

 0-75-0-85 mm. in diameter. Sac-like protrusions from nuclear mem- 

 brane, f Stage 6: Eggs i-8 mm. in diameter. Chromosomes and nucleus 

 much contracted. Central 'cloud' of nucleoli. Heavy arrows indicate 

 mixing of nuclear material in cytoplasm after germinal vesicle membrane 

 breaks down. Dotted arrow indicates migration of central chromosomal 

 mass towards the animal pole, where ist polar body maturation spindle 

 will form. {By courtesy, Ann. N.T. Acad. Sci.) 



prove to be more readily accessible to optical investigation. Discussion 

 about the nature of these chromosomes closely parallels that concern- 

 ing the salivary chromosomes. They are regarded as polytene by 

 PainterI^"; Ris^^° considers that they are wholly composed of coiled 

 chromonemata, while Dodson^^^ and Duryee^^^ regard them as 

 chromomeric. 



98 



