Figure 3
From: Placental expression of pituitary hormones is an ancestral feature of therian mammals
Growth and development of the tammar embryo and fetus. (a) At day 18, the conceptus emerges form the ruptured shell coat (Sh) so that the two regions of the placenta can make a close attachment to the uterine epithelium. The right side of the embryo is already coat-free, while the embryo itself is still within the shell coat. (b) The d18 embryo free of the fetal membranes, clearly distinguishing between the vascular and non-vascular regions of the yolk sac (BYS: bilaminar yolk sac; TYS trilaminar yolk sac; ST sinus terminalis). (c) Fetus at day 23 of pregnancy, showing the increase in the vascular region and the close attachment of the placenta to the uterine epithelium (Ut). The vitelline vessels are prominent. (d) Full term fetus at day 25 of pregnancy, about a day before birth. The allantois (All) is large but held within the folds of the yolk sac, which has become highly vascular. The fetus has well developed fore limbs ready for the climb to the pouch and the tongue is typically protruded (Am: amniopore).