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THE NEW INDIVIDUAL Part IV 



Skin Fore brain Future lens Optic vesicle 



Brain nearly closed 

 Future lens Future retina 



Brain and cord closed 



Future retina 



Lens 



Cornea 



Skin layer over lens becomes cornea 



Fig. 19.11. The development of the eye. Diagrams of cross sections of frog em- 

 bryos showing successive stages of the part taken by the ectoderm in the develop- 

 ment of the eye. Except for its part in the sense organs, the superficial ectoderm 

 becomes skin. A, embryo with brain open as in Figure 19.8, stages 14 and 15. B, 

 on each side of the head an outgrowth of the brain (optic vesicle) approaches the 

 lens, a thickened plate (placode) in the superficial ectoderm. C, the optic vesicle 

 at first shaped like a hollow dumbbell is now a shallow cup. The lens bends toward 

 the cup. The neural folds have closed and the future skin is separated from the 

 future brain. D, the lens has separated from the future skin ectoderm. The bottom 

 of the optic cup (vesicle) is the future retina. The lens nearly fills the top of the 

 cup. The superficial ectoderm outside the lens will be the cornea. 



from the skin ectoderm which later becomes the cornea (Fig. 19.11). The 

 accessory parts of the eye, the coats, blood vessels and muscles, are developed 

 from mesoderm. 



The sensory parts of other prominent sense organs, inner ears, nose and 

 taste all develop from ectoderm in fundamentally similar ways. The lateral 

 line system consists of a series of sense buds arranged in rows over the head 

 and body. Each line begins as a thickening of sensory ectoderm which later 

 breaks up into the sense buds that respond to vibrations in the water. Lateral 

 lines are conspicuous in bony fishes and in tadpoles, but they do not persist in 

 frogs and toads. 



Digestive System. As the body grows longer, the enteron also lengthens and 



