chloroplasts were simply innate “organs” of each cell, which had “gradually differentiated” out of the otherwise colorless cytoplasm. That was the endogenous theory: chloroplasts had taken shape on the inside of plant cells, formed from internal materials. Not so, he argued. Rather than being homegrown organs, they are “foreign bodies, foreign organisms” that invaded the cytoplasm of animal cells sometime in the distant past and entered into a symbiotic coexistence. According to this theory, a plant cell is nothing but an animal cell with photosynthetic bacteria added.
