The mechanism of interspecies surrogacy is outlined in the article linked. I first came across this concept a few days ago, and the ethical implications behind it have perturbed me since. There are several levels by which we could look at this issue and it's potential, and each level is accompanied by it's own crop of complications.

First, interspecies surrogacy would provide a cheap way for infertile humans to reproduce and for endangered animal populations to be revitalized. However, both would be at the expense of the surrogate, here assumed to be a non-consenting animal. We already gestate animals with fetuses of their own species- would implanting embryos of other species be taking it too far?

Second, interspecies surrogacy could tie in with genetically modified fetuses on a few levels. Babies born from a larger animal would have had more room to grow in utero, and could grow to be stronger and more viable than their natural counterparts. Furthermore, in order to preserve the fetus's life, we would need to suppress the surrogate's own immune system so that it doesn't reject the fetus.

There may be more discussion to be had, but these are my primary thoughts on the matter. If anyone can educate me further I would be grateful!