By a masterful thought experiment, Sturtevant demonstrated the power of applying genetics to embryology. To do this, he applied Mendelian genetics to the study of snail coiling.
Sturtevant, A. H. 1923. Inheritance of direction of coiling in Limnaea. Science, New Series, 58: 269–270.
Link:
http://www.esp.org/foundations/genetics/classical/holdings/s/ahs-23a.pdf