why did Tennessee Williams name this play "The Glass Menagerie"?

In the play Laura has her own private collection of glass animal dolls and the play is called "The Glass Menagerie" after Laura's collection of glass animal dolls. In Scene 7 Laura shows Jim, the Gentleman Caller, her collection and remarks,

"Most of them are little animals made out of glass, the tiniest little animals in the world. Mother calls them a glass menagerie!"

The glass dolls symbolize Laura's own fragility. Laura is crippled and they comfort her as well as remind her of how tenuous life can be.