The serpent's egg
Brutus:
And since the quarrel
Will bear no color for the thing he is,
Fashion it thus: that what he is, augmented,
Would run to these and these extremities;
And therefore think him as a serpent's egg,
Which, hatch'd, would as his kind grow mischievous,
And kill him in the shell
Brutus, soliloquizing, casts about for a rationale to join the conspiracy to assassinate Caesar [see MASTERS OF THEIR FATES]. The problem is that, so far, Caesar has not taken much advantage of his new power. "The thing he is"—a reasonable, stable leader—lends no "color," or credibility, to the argument that he is a danger to Roman liberties.
In a passage remarkable for its feeling of spontaneous thought, Brutus proceeds by extrapolation. Take what Caesar is now, he argues, augment it, and the result looks more threatening. Caesar is a "serpent's egg," a tyrant waiting to be hatched. If we find the egg, we recognize the "kind" (species); when the kind is dangerous, better to kill the creature in the shell than let it hatch to perform its mischief. The equivalent idiom today is "let's nip this problem in the bud."