Development of Sense Organs of Animals
Lamellated corpuscles are differentiated from a mass of mesenchymal cells clustering around a nerve termination. These multiply, flatten and give rise to concentric fibrous lamellae, the Pacinian corpuscles. A tactile corpuscle originates with a looping plexus of terminal nerve fibres located just beneath the epidermis and this plexus becomes encapsulated along with a cluster of mesenchymal cells.
Local thickenings of lingual epithelium present taste buds. The basal cells gets elongated and extend towards the surface and specialized into taste cells ending in hair like tips, while others become columnar supporting cells. The gustatory cells are connected to the nerve terminations.
The first indication of the olfactory epithelium is the thickening of ectoderm of the ventro-lateral surfaces of the head olfactory placodes. These are concerned into olfactory pits which elongate and from the nasal cavities. Rupture of the plate at the deep end of the olfactory sac produces the primitive choanae or posterior nares.
DEVELOPMENT OF EYE
- The eye is derived from three sources
- The optic nerves and retina from the forebrain
- The lens from the ectoderm of the head and
- The accessory tunics from the adjacent mesenchyme.
From the floor of the forebrain on each side, a diverticulum develops as the optic vesicle. This vesicle elongates to from the optic stalk and it terminates by widening out to form the optic cup. The optic cup is formed by indentation of its distal wall brought about by rapid marginal growth. The optic stalk forms optic nerve and the cup forms the anterior wall forming the nervous portion and the posterior wall, pigmentary layer.
Meanwhile the surface ectoderm overlying the optic vesicle thickens into a lens placode and this pockets inwards to produce a lens vesicle, which occupies the cavity of the optic cup. The lens vesicle has a layer of cubical cells in front forming the epithelium of the lens and the cells of the back wall from elongated lens fibres.
The mesoderm surrounding the optic cup forms the sclera, cornea and the chorioid. The vitreous humour is a secretory body from both retina and lens. Later probably and invading mesenchyme also contributes to its structure.
DEVELOPMENT OF EAR
The external ear is derived from ectoderm. A series of small swellings develop around the first two branchial grooves called the auricular hillocks give rise to the external.
The middle ear is derived from endoderm. The dorsal projection of the first pharyngeal pouch retains its proximity to the otocyst and becomes the lining of the eustachian tube and the middle ear cavity.
The internal ear is derived from ectoderm. A thickened ectodermal plate, the auditory placode located on each side of the hindbrain becomes the auditory pit, which soon close into the otocyst. These sink into the mesoderm and get detached from the ectoderm. From the otocyst, the membranous labyrinth and neuroepithelium are developed.
DEVELOPMENT OF SKIN
The integument has a double origin. The epidermis is derived from ectoderm and the dermis or corium from the mesoderm.
From the ectoderm, nails, hoof, horn, hair, sebaceous glands, sweet glands, mammary glands and arrector pili muscle are developed.
