DEVELOPMENT OF THE SENSE ORGANS 383 



The Sclerotic and Chorioid Layers, and their Derivatives. — After the mes- 

 enchyme grows in between the ectoderm and the lens (Fig. 377), the lens and op- 

 tic cup are surrounded by a condensed layer of mesenchymal tissue, which gives 

 rise to the supporting and vascular layers of the eyeball. By condensation and 

 difierentiation of its outer layers, a dense layer of white fibrous tissue is developed, 

 which forms the sclerotic layer. This corresponds to the dura mater of the brain. 

 In the mesenchyme of 25 mm. embryos a cavity appears distally which separates 

 the condensed layer of mesenchyme continuous with the sclerotic from the vascu- 

 lar capsule of the lens (Fig. 379). This cavity is the anterior chamber of the eye 

 and separates the anlage of the cornea from the lens capsule. 



An inner layer of mesenchyme, between the anlage of the sclerotic and the 

 pigment layer of the retina, becomes highly vascular during the sixth month. 

 Its cells become stellate in form and pigmented, so that the tissue is loose and 

 reticulate. This vascular tissue constitutes the chorioid layer, in which course 

 the chief vessels of the eye. The chorioid layer corresponds fo the pia mater of 

 the brain. Distal to the ora serrata of the retinal layer the chorioid is differen- 

 tiated: (1) into the vascular folds of the ciliary ladies; (2) into the smooth fibers 

 of the ciliary muscle; (3) into the stroma of the iris. The proximal pigmented 

 layers of the iris are derived from the pars iridis retinae and from a corresponding 

 zone of the pigment layer. Of these, the pigment layer cells give rise to the 

 sphincter and dilator muscles of the iris. These smooth muscle fibers are thus of 

 ectodermal origin. 



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 Zi mm. embryos (Fig. 

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

 takes place. A third rudimentary eyelid, corresponding to the functional nicti- 

 tating membrane of lower vertebrates, forms the plica semilunaris. The epi- 

 dermis of the eyelids forms a continuous layer on their inner surfaces known as 

 the conjunctiva, which in turn is continuous with the anterior epithehum 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 epithehum, there are developed about 

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

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



The Lacrimal Glands appear in embryos of about 25 mm., according to 

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



