Development of the Anal Cavity
The anal cavity and lower part of the rectum in the fetus. Left figure is four month. Middle figure is six months. Right figure is 9 months. In each the anal canal is distinctly marked off from the rectum proper; the columns of Morgagni and the rectal valves are distinct.
Gray, Henry Gray's Anatomy: Descriptive and Applied (Philadelphia: Lea & Febiger, 1913) 1308