Middle

Before it was called Prospect Avenue, the street between 16th and 17th was Middle Street.  Opened in the 1850s, it extended from the Gowanus Bay up to the Flatbush city line.  The Times, reporting in 1867 on the founding of a German Protestant Church on Middle, between Fifth & Sixth, refers to the block as being in Gowanus - beyond today's generally accepted Fourth Avenue boundary.  The 1867 church was a wooden building, but its replacement is still there, up from the Grand Prospect Hall:

Down the hill, around Fourth, between Middle & 16th, the Vigilant and the E Pluribus Unum clubs played ballgames in the 1850s. This would have been somewhere around here:

 

And on November 19, 1868, right around this corner, the Times recorded an ugly incident:

Last evening, while James Boss, a colored man, residing in Fourth Avenue near sixteenth Street, was cleaning his employer's horse in the stable in Middle Street,near Fourth Avenue, he was assailed by two rowdies who beat him about the head until he was insensible. After recovering, he went home and said nothing about it, but subsequently feeling worse he complained and told the story. His recovery is now regarded as doubtful.

Middle Street was only around for one more year.  In 1869, Middle between Hamilton Av. and the city line was widened, & it became Prospect Avenue.  Eighty or so years later, the Expressway arrived:

Prospect Avenue at Fourth