304 Gene Action 



normal. If any serious disturbance to this pattern occurs, de- 

 velopmental processes following such a disturbance are no longer 

 normal, and growth and development thereafter fail to follow 

 the normal pattern. Such a disturbance may be genetic or it 

 may be environmental. 



The processes of development are many and complicated. 

 During the earliest stages, just after fertilization, various cur- 

 rents appear in the cytoplasm which initiate the whole devel- 

 opmental pattern and start development off in the manner of 

 that species. Such plasma flowings are described in detail by 

 Conklin for Crepidula plana. Modifications due to environ- 

 mental influences may change the normal currents and thus alter 

 normal development. Later development seems to be due to 

 chemical reactions between the genes of the two species and 

 the cytoplasm and may also be disturbed by unusual conditions 

 of the environment. Unless these conditions are extreme, how- 

 ever, development proceeds normally according to the pattern 

 of the species. Although all gene products are interacting at all 

 times, it is probable that the differences between two organisms 

 that are unlike by only one pair of alleles occur at the time the 

 organs specifically affected by those genes are developing. 



In the vestigial and wild-type fruit flies, the vg and vg+ genes 

 in both organisms are interacting with the other genes and with 

 the cytoplasm during the earliest stages of development, but up 

 to the time of wing formation and growth, no differences between 

 the two types of flies can be detected. When the wing is being 

 formed, however, these two genes are still interacting with other 

 genes and with the cytoplasm, but now the reaction is of such 

 a nature that the two types can be differentiated. During wing 

 formation, one gene affects the developing wang in such a way 

 that it remains small and poorly developed, whereas in a related 

 fly, even a sib, the other gene affects the developing wing so as 

 to cause it to develop "normally." Thus, while all genes are 

 probably producing gene products throughout development, the 

 difference between two alleles will not be obvious until a certain 

 organ has reached a certain state of development. The time at 

 which the action of a gene becomes noticeable phenotypically 

 differs with different genes. Some genes appear to act early be- 

 cause they affect an organ that develops early; other genes do 

 not come into prominence until much later. Some genes that 



