Mammals

When considering the Animal Kingdom the first major spit is between animals with backbones and those without. Those with backbones include fish, amphibians, reptiles, birds, and mammals. The next major split is between the amniotes and nonamniotes. This is an important split because amniotes (reptiles, birds, and mammals) are able to reproduce on land. Fish and amphibians have external fertilization and must lay their eggs in the water. Birds and reptiles lay eggs. Most mammals have their young develop inside their bodies, however there is one group that does not. Mammals can be separated by the way their young develop. All mammals have a few things in common. They all have fur, 3 inner ear bones, and feed their young milk. Mammals are separated into the subclasses protheria and theria. Theria is subsequently split into the infraclasses metatheria and eutheria.

Subclass Protheria

The protheria subclass is made up of Platypuses and 4 species of Echidna. These animals are also called monotremes and they lay soft shelled eggs. The monotreme egg is similar to reptile and bird eggs. Monotremes do not have nipples, however they do feed their young milk from pores on their skin. Monotremes are only found in Australia and New Guinea.

Monotreme Egg

I made this monotreme egg out of a paper Easter egg (chorion) I got from IKEA, a plastic bag (amniotic sac), and two balloons (yolk sac and allantois). I used a little rubber band to hold all the soft membranes together.

Subclass Theria, Infraclass Metatheria

Marsupials are in the infraclass metatheria. Their young develop inside the mother’s body, however the offspring must finish development in the mother’s pouch. Marsupials only live in Australia and the Americas. The only type of marsupial in the Americas is the opossum. In the distant past some opossums migrated to Australia and then they adapted to many available roles and diversified into the many species of marsupials found in Australia today.

Subclass Theria, Infraclass Eutheria

All living mammals native to Asia, Africa, Europe, Antarctica, and the Americas (except for opossums) are in the infraclass eutheria. Humans are also in this group. The offspring in this group develop completely inside the mother’s body. The same membranes in the eggs of monotremes, reptiles and birds are still present in therians, but they are in a different form. The shell in eggs is the chorion and in therians it, along with the amnion (the membrane immediately around the offspring) form the amniotic sac. The yolk sac provides nutrients in eggs, but in therians the placenta delivers nutrients from the mother to the offspring. The yolk sac is still necessary for healthy development of the offspring. The allantois gets oxygen to the offspring in an egg, and it is part of the umbilical cord in therians.

Model of Therian Internal Development

I crocheted this by first making the amniotic sac (blue) like you would a hat, then I crocheted the chorion (grey) around the edge. Next, I made the uterus by crocheting pink around the edge and then decreasing to cover the blue area. I then added the rest of the membranes: yolk sac (yellow), umbilical chord which is partly made up of the allantois (white), placenta (maroon).