ABSTRACT
1. During the first four days after pupation the eye anlage extends by cell proliferation from a dorsocaudal differentiation center. This extension is due to a peripheral growth zone and leads to a tripling of the eye area. During its extension, the eye anlage incorporates adjacent epidermal material. 2. Within this peripheral zone, which moves continuously outward, two concentric zones marked by dividing cells were observed (Fig. 1 c, 2a, b). After injection of tritium-labelled thymidine, the radioactive material is incorporated into these zones (Fig. 5). They are separated by an area in which no replication takes place. 3. In the course of development of the eye the mitotic rate decreases until pupation and then reaches its maximum at the end of the first day after pupation. Thereafter the mitotic rate decreases continously to zero on the 5th day (Fig. 3, Table 1). Most of the mitotic spindles are orientated radially with respect to the differentiation centre (180°), especially in the inner proliferation zone. Many of the remaining spindles were preferentially orientated tangentially (90°) with respect to the differentiation centre (Fig. 4). About 2/3 of the spindles are orientated parallel to the surface, the others are orientated in a diagonal or vertical direction (Table 2). 4. The3H-thymidine incorporation pattern indicates that in each zone most or all cells divide only once. 24 hours after injection the tritium-labelled nuclei of the inner zone were found in all regions of the praeommatidium; apparently all cell types which take part in differentiation of the ommatidium were labelled. The results are discussed with respect to the hypothesis of the clonal origin of the ommatidium from a stem cell.