THE HEREDITARY FACTORS AND DIFFERENTIATION 40I 



chromosomes of the nuclei react, by undergoing or not undergoing 

 diminution. 



A different but equally interesting method of chromosome 

 elimination is found in the fungus-fly Sciara coprophila. In this 

 species the cells of the male germ-line possess five pairs of chromo- 

 somes. In the somatic tissues of the male, only seven of these ten 

 chromosomes are found, one pair of large chromosomes and one 

 single member of another pair being eliminated. In the female the 

 somatic and probably the germ-cells contain eight chromosomes. 

 It is probable that here too elimination occurs, but extends to germ- 

 cells as well as to soma, and is confined to the pair of large chromo- 

 somes which is also eliminated in the male. There would then 

 exist not one but two types of chromosome elimination. It appears 

 that the decision as to which shall occur is predetermined in the 

 zygote by the genes in one particular chromosome of the mother. 

 Undoubtedly the reduced chromosome-complexes must differ from 

 each other and from the unreduced complex in their morphogenetic 

 and physiological effects, and the elimination process is thus here a 

 true link in the chain of differentiation. However, it seems certain 

 that this constitutes a highly exceptional method, but it is of interest 

 as showing that qualitative changes in the total gene-complex may 

 arise during early development in different parts of the embryo.^ 



§2 



The effect of external environmental factors, in co-operating with 

 the hereditary factors (and other internal factors) in producing de- 

 velopment, is shown by experiments in which embryos are made 

 to undergo development in abnormal environments. A simple and 

 striking case is that of sea-urchin eggs made to develop in sea- 

 water which is deficient in calcium. The blastomeres resulting from 

 the cleavage of such eggs do not remain in contact with one another, 

 but become separated as isolated and independent cells, so that 

 normal development of the original embryo is of course out of the 

 question (although each of the blastomeres of the 4-cell stage if re- 

 placed in normal sea-water can produce a normally proportioned 

 but diminutive larva)'^ (^gs. 44, 194). 



^ Metz, 1931. ^ Herbst, 1897, 1900. 



HEE 26 



