In terms of development mammals are weird. We have clearly evolved a set of wacky adaptations to allow us not only to mature entirely within the parent, but also to do so without the huge supply of yolk common to most other animals. Think of your regular egg. That big ball of yolk is covered by a single plasma membrane and provides everything that chicken needs for the roughly twenty-one days it takes to go from laying to hatching. As a freaky aside, that entire chicken (if fertilized) will grow from a small white mass (the blastodisc) visible on every single store-bought egg. Mammals, not having that whole bunch of rich and yummy stuff to eat in the weeks it usually takes them to develop, instead do a whole bunch of crazy things for which there is no real comparison elsewhere, except that a lot these structures (the amnion and alantois) are also present in the the eggs of birds and reptiles. What we’ve got here is a whole suite of adaptations used grow new organisms on land, which would be completely mind-boggling if you didn’t consider the 600 million years (that’s 219 billion days, to put it in perspective) that it has taken these systems to develop to their current state.