Differentiated stage of the male genitalia
Under the effects of androgen the differentiation and development of the male sexual organs become visible after the 3rd month.
The genital tubercle becomes longer and out of it forms the penis. The urethral folds also lengthen ventrally. Between these extends the urogenital sinus and forms the urethral groove, which is lined with endoderm. The floor of this sulcus thickens through epithelial proliferation and forms the urethral plate that temporarily fills it out.
Later a groove forms again and the two urethral folds fuse on the underside. This section will become the spongy part of the urethra, which for now terminates in a dead end in the anterior part of the penis.
In the rear part, the genital swellings transform themselves into the scrotum.
From the fused urethral folds an erectile mesenchymatous tissue, the penile spongy body, arises in the penis. At the distal penis section a ring-shaped furrow delimits the glans. Above the spongy body arise the two cavernous bodies (corpora cavernosa) and thus complete the penile erectile system.
The two genital swellings also fuse in the middle and form the scrotum. The line along which they fuse on the penis and scrotum is called the raphe mediana.
During the 4th month two ectodermal invaginations arise on the tip of the penis. First, a solid epithelial cord forms from the penis tip and binds itself with the dead-ended spongy part of the urethra at the level of the ring-shaped furrow. As soon as this epithelial cord has been canalized, one speaks in this section of the glandar urethra with the urinary meatus.
The two circular epithelial ingrowths, glandar lamella, form the prepuce that at the time of birth is still stuck to the glans but, during childhood, comes away from it.