A. Migration of the QR.p neuroblast (open oval) and its descendants. The QR.p neuroblast is generated in the posterior half of the animal and then migrates to about position 10. It then ceases to migrate and divides. One of the QR.p descendants undergoes programmed cell death (X) and the other migrates to about position 7. It then divides to generate the SDQR and AVM neurons (closed ovals). Figure 9A is based on Sulston and Horvitz . B. AVM positions in EA485 and TU2562. The average position for 100 AVMs in both EA485 and TU2562 was 6.3. The AVM positions were measured as described in Figure 1.