The number of follicle cells increases as oocytes grow. In Drosophila, for example, they increase from about 80 to over 1200. During yolk deposition the follicle cells do not divide, but insteat form a flattened epithelium. The proximal portion of an ovariole is called the vitellarium, which in mature insects is by far the greater part of an ovariole. Yolk deposition, or vitellogenesis occurs in the proximal part.
Oocytes vastly increase in size during vitellogenesis.
Yolk is categorized as protein yolk and lipid yolk. Juvenile hormone stimulates fat bodies to synthesize and export vitellogens, or yolk proteins. JH also causes gaps in follicular cells, a condition called patency by Ken Davy. This allows free exchange between oocytes and hemolymph, and the oocytes rapidly take up proteins from circulation. Lipid yolk is also derived from fat body.