An electron microscopic study was made of the developing sphincter and dilator muscles of irises of embryonal and neonatal rabbits. In 18- to 19-day-old embryos, the anterior epithelial layer consisted of a single row of cylindrical cells from which the sphincter and the dilator muscles were derived.
Tamura T, Smelser GK. Development of the Sphincter and Dilator Muscles of the Iris. Arch Ophthalmol. 1973;89(4):332-339. doi:10.1001/archopht.1973.01000040334016