104 



EMBRYOLOGY 



RETINAL 

 LAYER PIGMENTED 

 LAYER 



LENS FIELD 



CORNEA 



LENS 



PRESUMPTIVE LENS REMOVED 



LENS FORMS 



Fig. 51. The formation of the lens within the lens field. Top: The optic 

 vesicle grows out from the brain. The epidermis in this region constitutes a 

 large lens field. The lens develops at the point where the optic vesicle touches 

 the epidermis. Bottom: A large strip of epidermis including the presumptive 

 lens is removed. The lens still develops from the surrounding epidermis which 

 moves in to close the wound. 



Progressive differentiation within a field 



The characteristics of a field change as development goes on and as the 

 parts of a structure become determined in more and more detail. To illustrate 

 this point it is expedient to shift the discussion to the chick embryo because in 

 the chick embryo the isolated parts of an organ can be cultivated and will 

 differentiate. When the mesoderm of the hind-limb begins to proliferate and 



