In the past, researchers in embryology have focused largely on plucking choice stem cells from parts of an embryo that would grow into an animal and encouraging them to proliferate in glassware full of specially selected nutrients.
Be it mouse or moose, or humans or horses for that matter, placental mammals all start life in roughly the same way. Shortly after fertilization, the first cell splits until there are three basic domains of tissue: one that proceeds to create the animal itself, and two that contribute to organs that facilitate its growth inside the mother.
The step is a small one, equivalent to just a single day of development for an unborn mouse. But a lot can happen in that 24 hours of gestation. While the resemblance is quite significant in research, it is – so to speak – only skin deep, lacking the signals that would see it transform into the fully-formed organism it models.