The 3rd, named the corpus spongiosum (spongy body), is a tube that lies centrally beneath the many others and expands at the finish to form the tip of the penis (glans). The shaft and glans have no muscle mass fibers. The raised rim at the border of the shaft and glans is termed the corona. On days 15 to 28-the post-ovulatory stage, the Graafian follicle-now called the corpus luteum-secretes estrogen.