354 PRINCIPLES OF EMBRYOLOGY 



of the whole set of chromosomes would alter the effective balance between 

 the genes. The common occurrence of polyploidy in some form or other 

 in differentiated insect tissues does perhaps suggest that it is related to the 

 varying activity of the nuclei in the different tissues and a similar sug- 

 gestion has been made, chiefly with respect to plant material, by Huskins 

 (1947) and Huskins and Steinitz (1948). However it must be admitted 

 that most animals and plants which are wholly polyploid, and start 

 life with an abnormal number of chromosome sets in the fertilised egg, 

 do succeed in developing very normally and show little sign that the 

 effective balance of their genes has been altered. Moreover, Staiger and 

 Gloor (1952) have described a lethal factor (Ipl) in Drosophila hydei which 

 has a colchicine-like effect on the mitoses in the cells of the larva, and thus 

 leads to the formation of highly polyploid cells (up to 28-ploid). A similar 

 effect can be produced by cold shock treatment (Gloor 195 1). The dam- 

 aged cells in most cases eventually die, but there is evidence that brain 

 cells, for instance, may retain their differentiation and function as normal 

 constituents of the brain even when their chromosome number is con- 

 siderably larger than usual ; and there is no indication that as the chromo- 

 some number becomes altered they tend to assume some other histological 

 type. This makes it difficult to believe that the differentiation of tissues 

 in insects is directly related to ploidy. 



A remarkable example of nuclear differentiation has been described by 

 Lindahl (1953) in the echinoderms, in which the micromeres at the 

 vegetative end of the egg become haploid ; ^ in this case there is no evidence 

 to what extent this is a cause or a consequence of the differentiation of 

 that region of the embryo. Green (1953) fmds similarly that the mesen- 

 chyme of the tail tip in anuran tadpoles is haploid. 



A very peculiar situation appears to occur in mammals, in which there 

 is evidence that the chromosome number may vary from cell to cell in 

 the same tissue. In insects such as Drosophila, on the other hand, the loss 

 of one of the chromosomes from a cell always has a defmite effect on its 

 development. Perhaps the apparent ineffectiveness of abnormal chromo- 

 some numbers in mammals depends on an ability of substances to diffuse 

 from cell to cell, so that the gene-balance within a large mass of tissue is 

 more important than that within individual cells; or perhaps the abnor- 

 malities arise too late in development to have much influence. The ques- 

 tion requires much further study (see Beatty 1954). 



There is also some evidence of a more biochemical nature which indi- 

 cates a differentiation between the nuclei in various tissues (cf. Brachet 

 1952^, h). Thus ui Amphibia, Brachet has shown that the nitroprusside 



1 This has been denied by Makino and Alfort, 1954, Exper. 1 0, 489. 



