Read the poem by Anne Bradstreet.

Thou ill-formed offspring of my feeble brain,
Who after birth did'st by my side remain,
Till snatched from thence by friends, less wise than true,
Who thee abroad exposed to public view;
Made thee in rags, halting, to the press to trudge 5
Where errors were not lessened, all may judge.
At thy return my blushing was not small,
My rambling brat (in print) should mother call;
I cast thee by as one unfit for light,
Thy visage was so irksome in my sight; 10
Yet being mine own, at length affection would
Thy blemishes amend, if so I could:
I washed thy face, but more defects I saw,
And rubbing off a spot, still made a flaw.
I stretch thy joints to make thee even feet, 15
Yet still thou run'st more hobbling than is meet;
In better dress to trim thee was my mind,
But nought save homespun cloth, in the house I find.
In this array, 'mongst vulgars may'st thou roam;
In criticks hands beware thou dost not come; 20
And take thy way where yet thou are not known.
If for thy father asked, say thou had'st none;
And for thy Mother, she alas is poor,
Which caused her thus to send thee out of door.
What extended metaphor (a metaphor which is used throughout the poem) is the basis of the poem?