the manna in which genes act on development. This we ascribed to the fact 

 that the classical organisms of experimental embryology did not lend them- 

 selves readily to genetic investigation. Contrariwise, those plants and animals 

 about which most was known genetically had been little used in studies of 

 development. 



It would be worth-while, we believed, to attempt to remedy this situation 

 by finding new ways experimentally to study Drosophila melanogaster — 

 which, genetically, was the best understood organism of the time. Tissue 

 culture technics seemed to offer hope. In the spring of 1935 we joined forces 

 in Euphrussi's section of l'lnstitut de Biologie physio-chimique in Paris, 

 resolved to find ways of culturing tissues of the larvae of Drosophila. 



After some discouraging preliminary attempts, we followed Ephrussi's 

 suggestion and shifted to a transplantation technic. It was our hope that in 

 this way we could make use of non-autonomous genetic characters as a means 

 01 investigating gene action in development. 



Drosophila larvae are small. And we were told by a noted Sorbonne authority 

 on the development of diptera that the prospects were not good. In fact, 

 he said, they were terrible. 



But we were determined to try, so returned to the laboratory, made micro- 

 pipettes, dissected larvae and attempted to transfer embryonic buds from 

 one larva to the body cavity of another. The results were discouraging. But 

 we persisted, and finally one day discovered we had produced a fly with three 

 eyes. Although our joy was great with this small success, we immediately 

 began to worry about three points: First, could we do it again? Second, if we 

 could, would we be able to characterize the diffusible substances responsible 

 for interactions between tissues of different genetic types? And, third, how 

 many non-autonomous characters could we find? 



We first investigated the sex-linked eye-color mutant vermilion because 

 of the earlier finding of Sturtevant that in gynandromorphs genetically 

 vermilion eye tissue often fails to follow the general rule of autonomy (20). 



Gynandromorphs may result if in an embryo that begins development as 

 a female from an egg with two X chromosomes, one X chromosome is lost 

 during an early cleavage, giving rise to a sector that has one X chromosome 

 and is male. If the original egg is heterozygous for a sex-linked gene, say ver- 

 milion, and the lost chromosome carries the normal allele, the male sector 

 will be genetically vermilion, whereas the female parts are normal or wild type. 

 (Other sex-linked characters like yellow body or forked bristles can be used as 

 markers to independently reveal genetic constitution in most parts of the body.) 



s-78 



