This is waht I'm using to remember, anyone wanna try and pick it apart? (not a mnemonic but it will reduce the amount of work you have to do)
ecto - 'ecto' means outside so anything on the outside, plus the central nervous system (brain + spinal cord)
meso - Musculoskeletal system (including the tissue that connects), vascular system (including teh heart and the spleen because that's all red blood cells 😛), urogenital system (so that would mean kidneys/bladder and the reproductive organs), notochord
endo - everything else
c'mon that's only four things to memorize for meso i'd asy that's pretty good