Re: Science, medicine, and the future
The technology for animal-human chimeras, human embryo cloning, animal implantation of human embryos, exists for more than a decade. 
Nowadays, CRISPR-cas9 gene editing can quickly and successfully manage to engineer genetically modified farm animals that lack uterine/leucocytic surface proteins which induce rejections/abortions of xenographic embryos.
Several receptor mechanisms, specific for major histocompatibility complex, tolerize fetal antigen-specific maternal CD4⁺ T cells and CD8⁺ T cells, excluding fetal rejection, which can be effectively targeted. 
Only ethical dilemmas prevent embryonic gestational surrogacy into animal uteri. 
Competing interests: No competing interests