The dog’s internal structure and mammal reproduction

Mammal egg cells are very small with little yolk; they ripen in the ovary and are fertilised in the oviduct. The fertilised cell divides and the embryo attaches to the wall of the uterus. The embryo developing in the uterus is called a foetus; it is linked to the mother by the placenta and the umbilical cord: nutrients and oxygen pass from the mother’s blood to the foetus and waste products return. The mother’s and foetus’s blood do not mix; substances cross through the placenta wall. The length of pregnancy depends on the animal’s way of life: small animals that give birth in a nest or hole have short pregnancy and many blind young; fast animals living in the open have long pregnancy and bear 1–2 large young that can walk soon after birth. A mouse breeds 5–8 times a year, cattle once. Milk is formed in the mother’s mammary glands and comes out through teats; the number of teats depends on the number of young: 8–10 in a dog, 2 in a goat. Milk contains water, fat, carbohydrate, protein, vitamins and mineral salts. Mammals are close to reptiles in some features: skeleton, muscles and internal organs are similar, and the lower mammals have a cloaca and lay eggs. The ancestors of mammals were reptile-like animals (therapsids) with varied teeth and legs under the body, which lived more than 250 million years ago. The first small, warm-blooded mammals appeared about 200 million years ago; after the dinosaurs died out (about 66 million years ago) mammals spread across the whole Earth. Safety: drink only boiled or pasteurised milk; do not take a wild animal’s baby home as “abandoned” – the mother may be near.