II. THE PERIOD OF ORGAN DEVELOPMENT 



129 



Fig. 47. Diagram of the development of a vertebrate eye, (a) pro- 

 trusion of the eye-vesicles (o) from the brain (h); Z = lens primor- 

 dium; (b) invagination of the eye-vesicle, resulting in the formation 

 of the eye-cup; formation of the lens-furrow, (Z); (c) eye-cup still 

 connected with the brain through the narrow eye-stalk (o.s.); 

 differentiation of the wall of the eye-cup into retina (r) and pigment 

 epithelium (p); lens (l) has become free from the epidermis (ep). 



After Kiihn. 



sides of the head. At this point, the ectoderm thickens, thereby 

 forming the lens-placode which then becomes invaginated, and, 

 as a lens-vesicle, disengages itself from the ectoderm. At the 

 same time, the eye-vesicle also invaginates, thereby forming 

 the double walled eye-cup (Fig. 47 b). The inner wall of the cup 

 becomes thicker, and develops into the retina; the outer wall 

 remains thin and forms the pigment epithelium surrounding 

 the retina (Fig. 47 c). Later the iris will arise from the edges 

 of the eye-cup, and the eye-stalk will develop into the optic 

 nerve which connects the eye with the brain. The transparent 

 cornea is formed by the skin overlying the eye. Finally, the 

 mesenchyme surrounding the eye also contributes its share. It 

 penetrates into the eye-cup and takes part in the formation 



Raven - Outline Physiologie 9 



