Stage 1 polyps are characterized by the appearance of reddish pigment in the endodermal epithelial cells of the body column and a subtle swelling of the ectodermal layer over this region. This ectodermal swelling results from the aggregation and proliferation of interstitial cells. As Stage 1 progresses, these interstitial cells continue to proliferate and aggregate. Toluidine blue staining and BrdU pulse-labeling experiments demonstrate the increasing density of rapidly cycling interstitial cells in this area.

Stage 2 polyps possess an increased concentration of endodermal pigment and display an extensive swelling of the ectodermal layer due to the dense aggregation of interstitial cells. During this stage the aggregation reaches its maximum size, consisting of thousands of proliferating interstitial cells. The cells in the center of the egg field stop cycling and start differentiating into nurse cells. This differentiation process progresses radially from the center of the egg field. Nurse cell differentiation is characterized by an accumulation of lipid, glycogen, and yolk-like electron dense material (Ref. 2).

Stage 3 of oogenesis is marked by the first visible sign of the oocyte, which has the appearance of a large white spot located underneath the swollen ectoderm. Although most nurse cells have stopped cycling by this stage, some cells near the periphery of the aggregation remain mitotic. At this time the oocyte starts increasing its volume due to the incorporation of cytoplasmic fragments from the surrounding apoptotic nurse cells (Ref. 2), which are densely packed.

During stage 4 the oocyte forms finger-like extensions (Ref. 2) that engulf the surrounding nurse cells. These “pseudopods” often reach completely around the body column. Concurrent with phagocytosis, nurse cells develop morphological features of apoptosis and condense (Ref. 2).

During stage 5, the oocyte begins retracting its pseudopods and initiates a massive morphological change. The distal ends of the pseudopods thicken relative to the proximal parts as the center of the oocyte starts to enlarge. As the fingerlike pseudopods retract, nurse cells are drawn toward the center of the oocyte.

The oocyte has fully retracted its extensions by the end of Stage 6. The ectodermal layer continues to stretch as the oocyte  assumes an oblong shape

In the final stage of oogenesis, the oocyte breaks through the ectoderm and rests in a raised ring of retracted ectodermal tissue called the egg cup. This ectodermal retraction occurs in less than fifteen minutes as the egg squeezes through the cleft. The oocyte completes meiosis and extrudes the polar body at its distal end. At this time, the oocyte, which is ready to be fertilized, is full of spherical, compacted nurse cells that consist of dense chromatin masses and remnants of endoplasmic reticulum (Ref. 2). The diameter of a mature oocyte is approximately 0.5 mm. Due to the fragile nature of the mature oocytes, it was difficult to maintain them intact during mounting for microscopy. However, the egg cup can still be seen after the embryo has initiated cleavage (middle panel).