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 variation in size was due to increased heterochromatin content in quail embryos' nucleoli. Le Douarin utilized the Feulgen stain technique to differentiate between quail-derived and chicken-derived cells within her chimeric embryos by assessing the visible dye levels. The application of this staining method played a pivotal role in Le Douarin's research, allowing her to monitor specific quail cell fates during embryonic development.