Placodes , or epidermal placodes ( lat. Placodae ) - anatomical structures in the head of the embryo , originating from the ectoderm and forming at the site of its contact with the neural tube . Placodes are formed by thickening or \ and invagination of the ectoderm, migration of cells from it, or in a mixed way. From the placodes, the lens of the eye, the auditory vesicle , the olfactory epithelium are formed , in the primordial chordates, the organ of the lateral line .
According to the position relative to the body axes, placodes are divided into dorsolateral and epibrachial. Dorsolateral placodes located in the upper part of the embryo’s head (posterior part later) take part in the development of the head nerves , the auditory vesicle and its derivatives ( vestibular apparatus ), the lateral line organ ( mechano- and electroreceptors ). Epibrachial, located in the lower (subsequently anterior) part of the head of the embryo, partially give rise to the lens, olfactory epithelium and cranial nerves during development.
The lens develops when the optic vesicle contacts the ectoderm: part of the ectoderm invaginates and becomes the rudiment of the lens. In bird embryos , this process takes approximately seven hours.