Urethral groove
Encyclopedia
The primary urethral groove or urethral groove is a temporary linear indentation on the underside (ventral side) of the male penis
Penis
The penis is a biological feature of male animals including both vertebrates and invertebrates...

 during embryonic development.

In humans, it typically appears around 8 weeks of gestation and becomes closed into a normal male urethra
Urethra
In anatomy, the urethra is a tube that connects the urinary bladder to the genitals for the removal of fluids out of the body. In males, the urethra travels through the penis, and carries semen as well as urine...

by the 12th week.
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