Abstract
The formation of milk protein droplets was studied in the lactating mammary tissues of C3H/Crgl mice by means of the electron microscope. Small vesicular and tubular profiles measuring 100 to 300 A were found inside the sacs of the Golgi apparatus, and within closed vacuoles apparently derived from the Golgi membranes. The evidence indicates that the Golgi apparatus concentrates these small vesicular and tubular particles into typical milk protein droplets.