116 The Structure of Protoplasyn 



growth of the nuclei of secreting cells, giving six size classes 

 1 : 2 : 4 : 8 : 16 : 32, all of which are correlated with distinct phases 

 of secretory activity. 



It is, of course, well known that great variation in nuclear size 

 can occur, both normally and in experiments, without reproduction 

 of the constituent chromosomes or other permanent change in struc- 

 ture. Sometimes increase in nuclear volume is associated with 

 enlargement of the nucleolus. Metz and his students have shown 

 particularly clearly by in viva experiments that when hypertonic 

 Ringer solution is injected into the body cavity of Sciara larvae the 

 nuclear and chromosome volume of the salivary gland cells both 

 decrease by at least 25 per cent. With hypotonic solutions there is 

 an increase of at least 20 per cent. Complete recovery occurs. They 

 found also that reversible nuclear shrinkage occurs when cells are 

 under pressure. Upon asphyxiation of Sciara larvae with CO2 or N2 

 for 1-3 minutes, the nuclear volume of salivary glands remains 

 unchanged, but the chromosomes shrink and contract into a ball. 

 When shrinking they give out a clear fluid into the nucleus, and 

 when recovery occurs this is reabsorbed by the chromosomes. 



The regular alterations in nuclear size described above for many 

 animal tissues occur also in plants (Hoppner, 1939) . They are often 

 correlated with multiplication of the chromosomes within the nucleus 

 or of the chromonemata within the chromosomes. Berger (1938) 

 has reinvestigated the polyploidy long known to occur in mosquitoes. 

 He found that in Culex pipiens, with n r= 3 chromosomes, the ileum 

 of the larva grows without mitosis until the nuclei are 8-16 times the 

 normal volume. When mitoses begin, up to 48 chromosomes are 

 commonly observed, and some nuclei have 96, which is 32-ploid. 

 The chromosomes must therefore have divided three or four times 

 during the long larval development without division of the nucleus 

 occurring. Geitler (1938) called this phenomenon "endomitosis" 

 and found it to occur in many tissues of insects such as the Mal- 

 pighian tubules, testis septa, follicular epithelium, fat bodies, salivary 

 glands, connective tissues, and epithelium of the mid-gut. A particu- 

 larly high degree of polyploidy was found in the salivary gland 

 nuclei of Gerris. The X chromosome of Gerris is heterochromatic, 

 and the number of chromosomes sets could therefore be determined 

 by counting the heterochromatic bodies; thus the number of chromo- 

 somes can be estimated though the nucleus is not dividing mitotic- 

 ally. As high as 2,048-ploid nuclei were found. It is not known 

 whether in insects the polyploid tissues which divide mitotically 



