REPRODUCTION AND DEVELOPMENT 



567 



Nostril Forming 



Lens forming 



Outer ectoderm 



Lens vesicle Optic cup Auditory vesicle 

 Lens foils to develop 



Normal embryo 



OJI 



Cut out 

 source of 

 optic cup 

 and discard 



Lens fails to develop 



Cut out source of optic cup 

 and transplant beneath 

 ectoderm elsev/here. Optic 

 cup develops in nev/ location 



Ectoderm adjacent to 

 transplanted optic cup 

 is induced to form lens 

 vesicle 



Experimental embryo 



Figure 403. Experimental embryonic induction of the lens in an embryo. Diagrams are sdiema- 

 tic dorsal views of normal and experimental chordate embryos. 



Experimental embryology 



Embryonic induction 



It was possible to design instruments so 

 delicate that operations could be performed 

 on young embryos to dissect out and discard 

 that part of the brain wall which would 

 later form the optic cup (experimental em- 

 bryo, Fig. 403, right side). Such an opera- 

 tion constitutes an embryonic extirpation. 

 Would the ectoderm in that region form a 

 lens in the absence of the optic cup? It did 



not do so. But was this sufficient proof that 

 the development of the lens depended on 

 the passage of some influence from the op- 

 tic cup to the lens-forming ectoderm? Per- 

 haps the lens-forming ectoderm was merely 

 damaged during removal of the source of 

 the optic cup. To rule out this possibility 

 another type of experiment was indicated. 

 Suppose the source of the optic cup was re- 

 moved from its normal position and was 

 placed beneath the ectoderm elsewhere in 

 the body of a young embryo (experimental 



