490 



THE RETINA. 



vesicle, where the outer or posterior wall turns round to join the 

 inner or anterior. 



The ciliary muscle and the ligamentum pectinatum are both 

 derived from the mesoblast between the cornea and the iris. 



The Retina. At first the two walls of the optic cup do not 

 greatly differ in thickness. On the third day the outer or posterior 

 becomes much thinner than the inner or anterior, and by the 

 middle of the fourth day is reduced to a single layer of flattened 



p.Ch 



FIG. 289. SECTION OF THE EYE OF CHICK AT THE FOURTH DAY. 



e.p. superficial epiblast of the side of the head ; R. true retina : anterior wall of the 

 optic cup; p.Ch. pigment-epithelium of the choroid: posterior wall of the optic cup. 

 b is placed at the extreme lip of the optic cup at what will become the margin of the 

 iris. /. the lens. The hind wall, the nuclei of whose elongated cells are shewn at nl, 

 now forms nearly the whole mass of the lens, the front wall being reduced to a layer of 

 flattened cells d. m. the mesoblast surrounding the optic cup and about to form the 

 choroid and sclerotic. It is seen to pass forward between the lip of the optic cup and 

 the superficial epiblast. 



Filling up a large part of the hollow of the optic cup is seen a hyaline mass, the 

 rudiment of the hyaloid membrane, and of the coagulum of the vitreous humour, y. 

 In the neighbourhood of the lens it seems to be continuous as at d with the tissue a, 

 which appears to be the rudiment of the capsule of the lens and suspensory ligament. 



