Which statement about developing placental mammals is true?
A) The epiblast forms the amnion, which encloses the embryo in a fluid-filled sac.
B) The epiblast forms the yolk sac, which contributes to the formation of the chorion.
C) The allantois is only found in species in which the placenta does not readily remove nitrogenous wastes from the fetus.
D) Fetal and maternal blood directly mix at the placenta.
E) The amnion contributes to the umbilical cord, which connects the embryo to the placenta.

Question

Quizplus · Accepted Answer

The epiblast, a layer of cells in the blastula, forms the amnion, which encloses the embryo in a fluid-filled sac. This amniotic fluid helps protect the developing embryo/fetus from physical trauma and temperature changes.

Answer: D
 Fetal and maternal blood do not directly mix at the placenta. Instead, exchange of nutrients, gases, and wastes occurs across the placental membrane, which separates the fetal and maternal blood supplies. This exchange occurs via diffusion and active transport processes.

Answer: E
 The amnion does not contribute to the umbilical cord - however, both the amnion and the umbilical cord develop from the conceptus and are important structures for fetal development. The umbilical cord connects the fetus to the placenta, and contains blood vessels that transport nutrients and oxygen to the fetus, while removing wastes.

Which Statement About Developing Placental Mammals Is True