H. E. LEHMAN 205 



reduction from diploid to haploid is described as occurring be- 

 tween the 16- and 32-cell stage (Lindahl, 1953; however, con- 

 tested by Makino and Alfert, 1954); the remaining tissues in 

 these embryos retain the normal 2-N count. 



Without question the most striking examples of tissue-specific 

 chromosomal variation are to be found in insects where many 

 tissues possess specific multiples of the basic diploid number 

 characteristic for the species. The most detailed study is provided 

 by the work of Geitler ( 1934, 1941 ) on chromosome numbers in 

 various tissues of the "pond skater," Gerris lateralis. This species 

 is particularly favorable for study owing to the presence of dis- 

 tinctive heterochromatic regions on the sex chromosomes, which 

 permit them to be identified and to ascertain ploidy during inter- 

 phase. By this means Geitler found the chromosome number in 

 salivary gland nuclei to be 2048-ploid; Malpighian tubules were 

 from 32- to 64-ploids; and mid-gut epithelium and testicular 

 septa were octaploid. He further demonstrated that the nuclei 

 achieved their various polyploid states by endomitosis during 

 the development of germinal diploid cells, the individual chromo- 

 somes undergoing recognizable pro-, meta-, and anaphase changes 

 within the intact nuclear membrane. Somewhat comparable re- 

 sults have been reported for the grasshopper, Romulea (Mickey, 

 1945); the mosquito, Culex (Berger, 1938, 1941; Grell, 1946a,b); 

 the fungus fly, Sciara (White, 1946, 1947, 1948); and Drosophila 

 (Painter and Reindorp, 1939). The number of such cases prob- 

 ably could be extended considerably if it were certain that multi- 

 strand (polytene) chromosomes represent a preliminary step in 

 the establishment of endopolyploidy. Berger ( 1939, 1941 ) and 

 Grell (1946a,b), for example, have found that the diploid cells 

 of the mosquito intestine increase in size but do not divide dur- 

 ing larval development. During this period the chromosomes 

 duplicate themselves and form bundles of closely associated chro- 

 monemata (equivalent to 8-, 12-, 32-, or 64-ploid), which later 

 separate into individual chromosomes when division is resumed 

 at metamorphosis. If each strand in a polytene chromosome in- 

 deed represents internal chromosomal duplication, variations in 



