SENSE ORGANS. 169 



age-making instrument ; an organ perceiving light, but 

 not yet seeing objects. 



(4) In the next step, which is found in the nautilus, the 

 cup-shaped depression is closed in above until it becomes 

 a hollow vesicle with only a pin-hole opening atop. Now 

 for the first time we have an image, an inverted image, 

 on what is now plainly a retina (Fig. 107, d). Now for 

 the first time there is a perception not only of light, 

 but also objects. In a word, we have a true eye. But 

 the sight of objects is still imperfect, for it is only a 

 pin-hole image. 



(5) In the next step the pin-hole opening closes, but 

 the point of closure remains transparent as a cornea, 

 and the cavity or vesicle thus formed (optic vesicle) is 

 filled by secretion with a transparent refractive sub- 

 stance which may be regarded as a vitreous humor. 

 We have now for the first time a lens image, but yet 

 only a simple lens image (Fig. 107, e). This is the case 

 in the snail and many other gastropods. 



(6) Finally in the squid the last stage in this strange, 

 eventful history is found. In these there is a cutic- 

 ular ingrowth from the corneal surface which finally 

 separates as a crystalline lens (Fig. 107,/), and thus we 

 have a compound lens image. 



That these are really the steps of evolution of the 

 eye is proved by the fact that in embryonic develop- 

 ment the squid's eye passes through all these stages. It 

 is first seen as a dark spot, then as a saucerlike de- 

 pression, then as a cup-shaped depression, then as a 

 hollow cavity with a pin-hole aperture; then the aper- 

 ture closes and the vesicle fills, and, lastly, the crys- 

 talline lens is formed by cuticular ingrowth from the 

 cornea. This is the most perfect eye found among 

 invertebrates. 



In the invertebrate eye there is yet no chiasm (Fig. 



