The formation of the mouth around the mouthpiece and reed is called the embouchure.
More derived nematodes, nematomorphs, and tardigrades, which open the mouth at the anterior terminal pole, would have thus lost the original ventral formation of the mouth, probably related to its relatively late opening in development [ 19, 58, 71].
The formation of pore mouth plugs appeared to be the consequence of the spinning top shape of pores as well as of other structural features such as the size distribution of microcrystallites around the pore walls.
Besides, an interesting link between canonical Wnt signalling and modulation of basement membrane components, such as laminins and fibronectin, has been shown during the formation of the mouth in Xenopus (Dickinson and Sive, 2009).
P. caudatus reproduces by external fertilization, and the small embryos undergo holoblastic radial cleavage, gastrulation by invagination and epiboly [ 35], and deuterostomic formation of the mouth [ 34], which are all considered to be plesiomorphic features in the Ecdysozoa [ 34, 36].
These cells might participate on the morphological changes during prism formation and early axis specification of plutei, because they were absent from the ventral ectoderm of C. subdepressus; a region that remained flat until the formation of the larval mouth.
We showed that NiCl2 "ventralizes" treated hemichordate embryos and induces the formation of a circumferential mouth (Röttinger and Martindale, 2011).
Moreover, the same region, if transplanted to another cell using microsurgery, is capable of inducing formation of an ectopic mouth [ 6].
The first signs of this event are observed at the time of mouth formation, with the appearance of actin-positive circular fibers around the introvert-trunk boundary.
Labeling for EdU-positive cells, together with cells expressing the oral marker foxA [ 34], showed that these populations are co-localized, and indicates that the asymmetric proliferation observed in the introvert at this stage occurs ventrally, at the region of mouth formation and nervous system development [ 34].
However, the process of bolus formation in the mouth is difficult to observe.
