FORMATION OF THE OPTIC CUP 105 



tial part, the retina, is a derivative of the neural tube. Even while the 

 tube is still an unclosed groove in the surface ectoderm, the beginnings 

 of the two retinae can be seen as a pair of dimples in the anterior portion 

 of its floor — the part destined to become the forebrain of the embryo. 

 As the lips of the neural groove approximate and fuse to close the 

 neural tube and push it beneath the surface ectoderm, these pits or 

 'foveolas opticas' (Fig. 37a) are each rotated through a right angle so 

 that they form a pair of bumps on the sides of the closed-in forebrain 

 (Fig. 37b). They rapidly expand as if blown up from the inside, and 

 each becomes a bubble of tissue attached to the side wall of the fore- 

 brain by a broad, very short, hollow stalk. 



Fig. 37 — Formation of the optic vesicles. 



a, cross section of anterior portion of frog neural groove, as yet unclosed, showing foveolie 

 opticce. Redrawn from Eyclesheimer. 



b, cross section of head of 4mm. human embryo, after closure of the neural groove — the 

 foveolae now form the optic vesicles. Redrawn from Mann. 



/- foveolae opticEc; fb- embryonic forebrain; m- mesoderm; tie- neural ectoderm; /- optic 

 stalk; se- surface ectoderm; v- optic vesicle. 



At this stage the bubble of forebrain tissue is in contact with the sur- 

 face ectoderm of the side of the head and is known as the optic vesicle, 

 its connection with the forebrain proper being called the optic stalk. The 

 stalk slowly shifts its root backward as the brain becomes serially con- 

 stricted into five chambers, and is eventually connected with the second 

 of these, the diencephalon or tween-brain. 



Two processes now set in, one in the optic vesicle and one in the 

 surface ectoderm, which go on simultaneously and look superficially as 

 though one of them must be causing the other : an indentation of the 

 optic vesicle to form a two-layered optic cup; and an in-sinking of a 

 portion of the surface ectoderm to form a closed hollow ball of tissue, 

 the lens vesicle, which comes to lie in the cavity of the optic cup. The 



