I82 



STAGES OF DEVELOPMENT IN THE EYE 



CHAP. 



find that they represent stages in a general course of develop- 

 ment. Thus in Patella the eye is scarcely more than an invagi- 

 nation or depression in the integument, which is lined with 

 pigmented and retinal cells. The next upward stage occurs 

 in Trochus^ where the depression becomes deeper and bladder- 

 shaped, and is filled with a gelatinous or crystalline mass, but 

 still is open at the top, and therefore permits the eye to be 



op.n ' - - "(--' u 



Fig. 88. — Eyes of Gasteropoda, showing arrest of development at successive stages: 

 A, Patella; B, Trochus ; C, Turbo ; D, Murex; ep, epidermis; I, lens; op.7i, optic 

 nerve; ?•, retina; u./i, vitreous humour. (After Hilger.) 



bathed in water. Then, as in Turho^ the bladder becomes closed 

 by a thin epithelial layer, which finally, as in some Murex^ be- 

 comes much thicker, while the ' eyeball ' encloses a lens (Fig. 88), 

 which probably corresponds with the ' vitreous humour ' of other 

 types. 



In Nautilus the eye is of a very simple type. It consists of 

 a cup-shaped depression, with a small opening which is not 

 quite closed by the integument. The retina consists of cells 



