Investigators have disagreed widely as to the relative size of the muscularis of the pylorus in the fetus, in the full term infant, and in the infant with pyloric stenosis. In their studies of the fetal pylorus Erich Moeller,1 Jonnesco,2 and A. Retzius3 found the muscularis exceptionally prominent. In the fetus there is no pyloric valve as there is in the adult, and closure occurs by a contraction of the entire pyloric segment, which is known in the anatomic literature as the canalis pylori. They found the proximal boundary of the pyloric canal more or less indistinct, whereas the distal demarcation is usually rather sharp. In three premature infants Ibrahim4 reported the muscularis of the pyloric canal thicker than that of the full term normal infant. Wachenheim5 came to the conclusion that the diameter of the fetal pyloric musculature is often greater than that of