(…) these were actual mesh cages suspended from apartment windows, much like a window-unit air conditioner would be today (…) it was believed that babies who are exposed to daylight during the afternoon hours slept better than those who were not (…) Invented in the United States in 1922, baby cages really took off in London in the 1930s and allowed for city-dwelling moms to offer their young ones a bit of fresh air when heading down to the local park just wasn’t an option (…) The cages became popular in London in the 1930s among apartment dwellers without access to backyards (…) Once installed, a caretaker could simply place their tot inside the wire basket and go about tending the home

Excerpts available here and here