GENETICS — MORGAN 365 



at different levels. The fate of the cells in each region is determined, 

 it was said, by the differences in different protoplasmic regions of 

 the egg. 



Such a view is consistent with the idea that the genes are all acting ; 

 the initial stages of development being the outcome of a reaction be- 

 tween the identical output of the genes and the different regions of 

 the egg. This seemed to give a satisfactory picture of development, 

 even if it did not give us a scientific explanation of the kind of reac- 

 tions taking place. 



But there is an alternative view that cannot be ignored. It is con- 

 ceivable that different batteries of genes come into action one after 

 the other, as the embryo passes through its stages of development. 

 This sequence might be assumed to be an automatic property of the 

 chain of genes. Such an assiunption would, without proof, beg the 



Non-lerminal Deficiency ^Vf- 



"Plexa f e" heterozygote ^ES" ^ ' "chromoVo'mT 2 



End of normal 



Section involved- 



-DEFtCIENCV 



k lO/t — ►! '^^^M 



I I I I I I 1 I I I I '^n^r 



34d6 C.B.B. g^Bg! 



Figure 8. — Salivary gland preparation showing a part of cliromosome 2. There is a defi- 

 ciency in one of the two conjusants. At the level of the deficiency the other com- 

 ponent is bent outward, so that above and below this level like bands meet. This 

 figure also shows that the salivary chromosomes are made up of 16 strands, the 16 

 elements of which fuse together to make each of the cross bands. (After Bridges.) 



whole question of embryonic development, and could not be regarded 

 as a satisfactorj^ solution. But it might be that in different regions 

 of the egg there is a reaction between the kind of protoplasm present 

 in those regions and specific genes in the nuclei ; certain genes being 

 more affected in one region of the egg, other genes in other regions. 

 Such a view might give also a purely formal hypothesis to account 

 for the differentiation of the cells of the embryo. The initial steps 

 would be given in the regional constitution of the agg. 



The first responsive output of the genes would then be supposed 

 to affect the protoplasm of the cells in which they lie. The changed 

 protoplasm would now act reciprocally on the genes, bringing into 

 activity additional or other batteries of genes. If true this would 

 give a pleasing picture of the developmental process. A variation of 

 this view would be to assume that the product of one set of genes is 

 gradually in time overtaken and nullified or changed by the slower 

 development of the output of other genes, as Goldschmidt, for ex- 



36923—36 24 



