Figure 12-11. Diagram showing the orientation of an 8-day embryo in the uterus and of the planes in which sections are cut. Plane A: transverse to uterus, sagittal to embryo. In the early egg cylinder stages this may be referred to also as a longitudinal section of the egg cylinder. The orientation of the embryo is not always consistent and may sometimes depart by as much as 45° from this plane. Plane B: transverse section of embryo. Note that in embryos past the egg cylinder stage this plane, though transverse to head and tail regions, is frontal with respect to the midtrunk region. Plane C: frontal section of embryo. Note that in embryos past the head cylinder stage this plane through frontal to head and tail regions is transverse with respect to the midtrunk region. In early egg cylinder stages this may be referred to also as a longitudinal section of the egg cylinder.