Rh factor is a protein found on the outside of Rh-positive red blood cells. Rh incompatibility during pregnancy occurs when an Rh-negative mother is pregnant with an Rh-positive fetus. During delivery, the fetus' Rh-positive blood is introduced into the mother’s body. The Rh-negative mother’s body begins to produce antibodies that attack and kill Rh-positive blood cells. Since the crossover of blood normally occurs during delivery, an Rh-negative woman’s first pregnancy is normally not affected. However, the woman’s antibodies may attack a second Rh-positive fetus as those antibodies can cross into the bloodstream and destroy fetal Rh-positive blood cells.

Illustration of the animal-vegetal gradient in Xenopus laevis ( African clawed frog) eggs after fertilization. During fertilization, the sperm s point of entry determines the future dorsal side (shaded) and ventral side (unshaded) of the embryo. The prospective ventral side of the embryo forms on the side where the sperm enters while the prospective dorsal side forms opposite the sperm s point of entry. The animal pole (dark green), marginal zone (lime green) and the vegetal pole (yellow - green) are delineated here as the animal-vegetal gradient is determined in the egg before fertilization. The blastopore groove forms across from the sperm s point of entry (roughly 180 degrees).

In 1935, George Beadle and Boris Ephrussi developed a technique to transplant optic discs between fruit fly larvae. They developed it while at the California Institute of Technology in Pasedena, California. Optic discs are tissues from which the adult eyes develop. Beadle and Ephrussi used their technique to study the development of the eye and eye pigment. (1) The experimenter dissects a donor larva, which is in the third instar stage of development, and removes the optic disc (colored red) with a micropipette. Because the antenna disc is attached to the optic disc, they are often removed and transplanted together. (2) The experimenter then implants the optic disc into a host larva, in the part of the host that will develop into an adult abdomen. As the host larva matures to adulthood, the implanted optic disc develops into an eye inside the body cavity of the adult. (3) The adult host has an eye within its body, which Beadle and Ephrussi found by dissecting the adult hosts. If the antenna disc was also transplanted, sometimes the resulting eye developed with an antenna attached.