Lacking any scientific means of pinning down the soul, the first anatomists settled on generative primacy. What shows up first in the embryo must be most important and therefore most likely to hold the soul. The trouble with this particular avenue of learning, known as , was that early first trimester human embryos were difficult to come by. Classical scholars of ensoulment, Aristotle among them, attempted to get around the problem by examining the larger, more easily obtained poultry embryo. To quote Vivian Nutton, author of , analogies drawn from the inspection of hen's eggs foundered on the subject that man was not a chicken.