Final answer:
Mouse Hox genes, essential for embryonic development and determining the body plan in animals, have been most likely activated within mouse embryos themselves.
Step-by-step explanation:
The question relates to the field of developmental biology and specifically deals with Hox genes, which are critical in determining the course of embryonic development across vertebrates. The study of Hox genes has shown that they are highly conserved and play a significant role in dictating the general body plan and structure in developing embryos. For example, Hox gene mutations can lead to the kinds of embryonic transformations that might result in legs where antennae should be in fruit flies, which reflects the key roles these genes play in segment-specific development.
Mouse Hox genes, analogous to those found in humans, have been activated experimentally in the embryos of other developing animals. However, the question appears to relate to manipulations or comparisons within vertebrate embryos. Given the nature of Hox gene conservation and expression in development, as described in the given reference, these genes were most likely activated within mouse embryos, as they contain the same clusters of Hox genes – Hox-A, Hox-B, Hox-C, and Hox-D – that determine various aspects of the mouse's developmental process.