The shape of eggs varies considerably across bird species, ranging from near-spherical (such as those of the little bee-eater) to highly or conical (such as those of the common murre) with the familiar shape of the chicken egg lying in between. Early scientific investigators of egg shape suggested that the oval shape of eggs was caused by the egg being forced through the oviduct by peristalsis. In this often-repeated but incorrect theory of egg shape formation, the contraction and relaxation of the muscles which push the egg down the oviduct cause the spherical egg membrane to distort slightly into an ovoid shape, with the blunt end caudal (i.e. furthest down the oviduct and closest to the cloaca). The calcification of the egg in the shell gland/uterus then fixes it in this shape, and the egg is laid with the blunt end appearing first (for a diagram showing the different sections of the bird oviduct, see here). However, this theory has been refuted by studies of egg shell formation in a number of bird species using techniques such as X-ray photography which have demonstrated that egg shape is determined in the oviduct isthmus (before shell calcification) with the pointed end caudal (furthest down the oviduct). These observations cannot be explained by peristalsis. It has been proposed that the egg acquires its shape (with the pointed egg caudal) as it is forced through the narrow isthmus, but this assertion has not been thoroughly verified.