DEVELOPMENT OF THE SENSE ORGANS 381 



The Eyelids appear as folds of the integument in 20 mm. embryos. The lids 

 come together and the epidermis at their edges is fused in 33 mm. embryos (Fig. 

 360). Later, when the epidermal cells are cornified separation of the eyelids 

 takes place. The epidermis of the eyelids forms a continuous layer on their 

 inner surfaces as the conjunctiva, which in turn is continuous with the anterior 

 epithelium of the cornea. 



The Eyelashes, or cilia, develop like ordinary hairs and are provided with 

 small sebaceous glands. In the tarsus, or dense connective tissue layer of the 

 eyelids, which lies close to the conjunctival epithelium, there are developed about 

 30 tarsal (Meibomian) glands. These arise as ingrowths of the epithelium at the 

 edges of the eyelids, while the latter are still fused. 



The Lachrymal Glands appear in embryos of 22 to 26 mm., according to 

 Keibel and Elze. They arise as five or six ingrowths of the conjunctiva, dorsaUy 

 and near the external angle of the eye. The anlages are at first knob-like and 

 rapidly lengthen into solid epithelial cords. They begin to branch in 30 mm. 

 embryos. At stages between 40 and 60 mm. additional anlages appear which 

 also branch. 



In 38 mm. embryos a septum begins to divide the gland into orbital and palpebral por- 

 tions. This septum is complete at 60 mm., the five or six anlages first developed constituting 

 the orbital part. Lumina appear in the glandular cords in embryos of 50 mm. by the degener- 

 ation of the central cells. Accessory lachrymal glands appear in 30 cm. fetuses. The lachrymal 

 gland is not fully developed at birth, being only one-third the size of the adult gland. In old 

 age marked degeneration occurs. 



The Naso-lachrymal Duct is formed as a solid epithelial outgrowth from the 

 conjunctiva of the lachrymo-nasal groove at the internal angle of the eye. The 

 anlage grows down through the mesenchyme to the nasal cavity. The lachrymal 

 canals are budded out from the solid anlage of the lachrymal duct and become 

 connected secondarily with the inner margins of the palpebrae. The primitive 

 connection of the lachrymal duct with the conjunctiva is lost. The anlage of 

 the duct appears in 10 mm. embryos and in 25 mm. embryos has not yet reached 

 the nasal cavity. A lumen appears in the duct during the third month. 



The Development of the Ear 

 The human ear consists of a sound-conducting apparatus and of a receptive 

 organ. The conveyance of sound is the function of the external and middle ears. 

 The end organ proper is the inner ear with the auditory apparatus residing in the 

 cochlear duct. Besides this acoustic function the labyrinthine portion of the inner 

 ear acts as an organ of equilibrium. 



