The morphogenetic field is a psychobiological theoretical construct wherein a cell has the ability to contain metadata about the entirety of an organism. Often referred to as "holographic anatomy", it's an as-yet unproven area of research. Very popular area of argument in mutant biology, especially when dealing with shapeshifters and folks like Mondo, Jan, or Catseye.

In effect... well, yeah. Jennie's been turned into a dog. Trying to undo it would require a re-establishment of the original morphogenetic field pattern, which isn't something modern medical science can replicate mechanically - it'd be like trying to have a backup of someone's thought patterns to restore their brain after a stroke. Theoretically, whatever caused this should be able to undo it.