The Effects of Sexcombless on the Forelegs of Drosophila Melanogaster.

role of genes in differentiation has long been recognized. That there must T! f a discipline that guides the nonrandom differentiation of patterns is obvious from one simple fact. that the pattern is reproducible in every generation. It is evident that at certain developmental stages specific cells or cell areas are singled out from the rest to differentiate and thus form a pattern of differentiation. An underlying cause of this specific pattern was seen in existence, previous to the differentiation, of a nonuniform distribution of properties termed “prepatterns” (STERN 1954, a, b). No specific definition of the nature of prepattern was given, leaving i t open whether they have a morphodynamic identity or provide simple stimuli such as might result from specific physical o r chemical properties. One may then ask. is there any way to conceive a mechanism by which a prepattern lays a foundation for the realized pattern? This question must ultimately be answered in biochemical terms. Already tissue specific patterns have been measured at the protein level as for instance in respect to isozyme patterns in the lactate dehydrogenase (LDH) system of mice (MARKERT and URSPRUNG 1962). It has been suggested the steps preceding formation of LDH may serve as examples of prepattern for the final pattern of the enzyme system (see (URSPRUNG 1963). Presumably, a consecutive series of prepatterns may be formed in the course of development until a final pattern is realized. In the course of evolution of a species a specific terminal morphogenetic pattern might be lost by a single mutation but the prepatterned site might still be there. The interalar bristle, for example, is present on the thorax of some Dipterans normally, but not present in Drosophila melanogaster. STERN (1956) has shown that the prepattern for this particular bristle is present in both normal and the mutant form, Theta, but they differ from each other in having different cellular competence to respond to the prepattern. A second example concerns the development of the wing in the chick embryo. Here, the fibulare, a bony structure normally absent in modern birds can be caused to appear by simple genetic or morphogenetic alterations ( H A M P ~ 1959; see WADDINGTON 1962). The present work involves a study of the effects of the gene sexcombless (sz)