Neural Development in Drosophila 
less, the homeodomain in cut, and the helix- 
loop-helix motif in da. In each case, the motif 
immediately suggests the likely mode of gene ac- 
tion, which may be tested experimentally. 
The existence of readily identifiable motifs in 
the majority of genes involved in neural develop- 
ment suggests that cells have a limited repertoire 
of mechanisms for essential regulatory functions. 
These include various signaling systems such as 
ligands, receptors, second messengers, and regu- 
lators of gene expression. Perhaps only a modest 
number of new tools had to be invented for the 
formation of the nervous system, and many of the 
cellular mechanisms in neural development may 
turn out to be ones already in use before the ner- 
vous system evolved. Understanding neural devel- 
opment may require an understanding of the 
usage and manipulations of these basic functional 
motifs. 
It is apparent that many of the functional motifs 
have been highly conserved during evolution. 
Several hundred million years separate the fly and 
vertebrates, yet the aforementioned motifs — EGF 
repeat, tyrosine kinase domain, etc. — are clearly 
conserved. Thus studying development in organ- 
isms with well-developed genetics, such as fruit 
fly and nematode, may provide not only reason- 
ably satisfactory answers but also useful tools 
for the study of corresponding problems in 
vertebrates. 
220 
