The influence of culture medium and protein supplements on in vitro fertilization (IVF) and “morula to blastocyst” development in culture was examined in the domestic cat. Even though cats aren’t our closest relative you can still see the similarity between the two mammals as embryos. Our tails normally go away by the time we’re born, but not always. The study of one type of evidence of evolution is called embryology, the study of embryos. An embryo is an unborn or (un-hatched) animal or human young in its earliest phases. Embryos of many different kinds of animals: mammals, birds, reptiles, fish, etc. look very similar and it is often difficult to tell them apart. Many traits of one type of animal appear in the embryo of another type of animal. For example, fish embryos and human embryos both have gill slits. In fish they develop into gills, but in humans they disappear before birth.