Final answer:
The young of placentals develop inside the uterus, a muscular organ capable of expanding during pregnancy and contracting during birth. The placenta provides nourishment from the mother's blood, and the vagina acts as the birth canal.
Step-by-step explanation:
For placentals, the young develop inside a sac-like organ called the uterus. The uterus is a pouch-like, muscular organ in which the embryo or fetus develops. This organ can expand greatly to accommodate a growing fetus, and its muscular walls contract during birth to deliver the baby. It contains a thick lining called the endometrium, where the placenta attaches and the amniotic sac, which protects the developing fetus, forms.
The vagina serves as a passageway for menstrual flow, copulation, and as the birth canal during the delivery of offspring. The young are nourished by the mother's blood through the placenta, a process known as viviparity, which occurs in most mammals as well as some other species.