Only one embryo develops within each uterus at a time, resulting in litters of two (rarely one) pups.

There are a number of differences, most notably in how the embryo develops.

As the embryo develops, these homologies can be lost to view, and the structures can take on different functions.

The embryo develops an egg tooth during incubation, which it uses to tear open the egg; the tooth disappears soon after hatching.

Within a few weeks of fertilization, the embryo develops rudimentary gonads (testes), which produce hormones responsible for the development of the reproductive system.