The Ultimate Weapon Takes Shape
Erico Fermi designed a device which could eventuate in chain reaction.
It consisted of layers of graphite, alternated with chunks of uranium.
The uranium emitted neutrons, and the graphite slowed them down. Holes
were left for long cadmium control rods. By withdrawing these control
rods, Fermi could speed up the production of neutrons, increasing 
the number of atoms which would be split. When the rods were withdrawn
to a critical point, the neutrons would be produced so fast that the 
graphite and cadmium couldn't absorb them. In this manner a chain reaction
would result. Slowly, Fermi's atomic pile begin to grow in a 
subterranean room at Columbia University. But the big question remained
--was it viable?

the lion's share--Because the salesman was essential to the business,
he demands the lion's share of the profits.
(the major portion)