Dark-field and electron microscopic studies of budding yeast cells have shown an extension of a wall of full thickness, rather than a break in the wall, when the bud emerges. The bud appears as a minute bulge and grows steadily, not explosively, during which time both it and the mother cell are enclosed within a single wall. The wall maintains essentially a constant thickness throughout the growth of the bud, and at maturity both the wall and the cytoplasm of the two cells are separated by a cleavage wall which is laid down between them.