Sixty-five pregnant rabbits were autopsied at different stages of pregnancy 2 to 28 days post coitum (p.c.). The vascularity of the endometrium and placenta was studied using intervascular injections of neoprene latex cast, by clearing injected specimens and by the study of frozen and paraffin embedded sections. At 6 days p.c. the implantation sites were identified; the endometrium at the inter-conceptus was similar to that during pseudopregnancy. The subepithelial capillary plexus increased in thickness, reaching a maximum at 6–11 days p.c. Marked endometrial branching occurred 10 days p.c. After 15 days p.c. the endometrium formed a thin irregular layer. Most of the endometrial epithelial cells showed cytoplasmic projections at 6 days p.c. and were transformed into syncytial epithelial cells 8 days p.c. At 24–28 days p.c. the conceptus and interconceptus areas were unidentifiable.

The size of the obplacental folds diminished at 6 days p.c., and the placental folds increased in size after 7 days p.c. At 9 days p.c. blastocyst attachment was observed histologically. The surface capillaries were dilated and changed into surface sinuses. Networks of blood pathways were established; the pathways connected with large veins in the core of the fold. The veins extended to the hemorrhage areas representing the origin of fetal placenta. The fetal vessels were recognizable near the base of the fetal placenta at 11 days p.c. The maternal circulation in the fetal placenta started before the fetal vessels reached the fetal placenta. At 10 days p.c. the perivascular sheath of the maternal vessels, in the placental folds, developed markedly as to replace most of the connective tissue. With advancing pregnancy the number of vessels within the maternal placenta decreased and some vessels increased in size. At 13 days p.c. the zone of separation was formed. At 15 days p.c. the lobes were formed in the fetal placenta; all placental elements were established anatomically. Subsequent stages were characterized by developmental changes in the fetal placenta and degenerative changes in the maternal placenta.