FIELDS AND THEIR PROPERTIES 103 



center the frequency of perfect ear vesicles which may be obtained will be 

 perhaps 80 or 90 percent. And at the very periphery of the field the incidence 

 would drop to zero. 



A field is larger than the structure which forms within it. Fields of 

 different structures actually overlap, so that tissue which normally forms a 

 lens can also form a nasal placode. Since a large field exists for each of these 

 various structures, certain factors must select from that large field the actual 

 structure itself. The question is, given a field such as the lens field, why is it 

 that the lens normally forms in its specific location with respect to the optic 

 vesicle? 



Let us first examine normal eye development in amphibians (Fig. 51) . In 

 the formation of the eye the brain forms, first of all, an outgrowth, which is 

 called the optic vesicle. This optic vesicle comes in close contact with the 

 overlying ectoderm, at this time called the epidermis. Where the optic vesicle 

 touches the epidermis a thickening appears, and this thickening develops 

 into the lens of the eye. In other words, at a somewhat later stage, when the 

 optic vesicle has differentiated into the retinal layer and the pigmented layer 

 of the optic cup, this thickening, the lens, lies in the opening of the optic 

 cup. The overlying ectoderm with some mesoderm then becomes converted 

 into the transparent cornea. Here is a contact relationship by which one 

 structure, the optic vesicle, is related to another structure, the lens. And this 

 contact relationship proves to be another case of embryonic induction. Just 

 as the contact of the roof of the archenteron with the ectoderm converts the 

 latter into a neural plate, so the optic vesicle causes the epidermis to form a 

 lens. 



Proof of such a lens induction comes from a simple type of operation in 

 which the lens epidermis of an embryo is removed from the optic vesicle 

 region (Fig. 51). The surrounding epidermis moves in from the sides and, 

 coming in contact with the optic vesicle, forms a lens. The epidermis in the 

 head region is induced to form a lens by the stimulus of the optic vesicle, and 

 thus the optic vesicle determines the exact position of lens formation in the 

 lens field. The induction of the lens by the optic vesicle is one of the oldest 

 and clearest examples of embryonic induction. In some species of amphibians 

 the lens fails to develop if the optic vesicle is removed. Thus the lens is 

 dependent upon the optic vesicle. In most species tested the optic vesicle, if 

 transplanted beneath the flank epidermis of a gastrula, will induce a normal 

 lens from this foreign epidermis. 



