The size disparity was caused by the tendency of quail embryos to carry increased amount of heterochromatin in the nucleolus. Le Douarin was able to utilize Feulgen stain to stain the heterochromatin and distinguish between groups of quail-derived and chicken-derived cells in her chimeric embryos based on the amount of visible dye. The use of Feulgen stain was an important turning point in Le Douarin's research, as she was now able to track specific quail cell fates in the developing embryo.
The size discrepancy was due to quail embryos having higher levels of heterochromatin in their nucleoli. Le Douarin utilized Feulgen stain to highlight the heterochromatin and differentiate between quail-derived and chicken-derived cells within her chimeric embryos by observing the quantity of visible dye. The application of Feulgen stain was a crucial advancement in Le Douarin's research, allowing her to monitor specific quail cell fates during embryonic development.