162 



DEVELOPMENT OF THE FROG'S EGG [Cii. XV 



cavity of the vesicle is nearly obliterated (Fig. 49) ; and at 

 the same time the inturned wall becomes greatly thickened. 

 There is thus formed an open, cup-shaped structure with the 



opening of the cup turned 

 outward. The wall of tins 

 optic cup lying toward the 

 brain remains thin, and pig- 

 ment soon appears in it. 

 The inturned wall becomes 

 the retina of the eye. 



At the time when the 

 optic bulb turns in on it- 

 self, a thickening of the 

 inner layer of ectoderm op- 

 posite the optic cup takes 

 place. This thickening 

 forms a solid mass of cells 

 projecting into the open 

 mouth of the cup. It be- 

 comes hollow and then sep- 

 arates from the ectoderm 

 (Fig. 49), filling up the 

 opening of the optic cup, 

 and forms later the lens of 

 the eye. In the space left between the lens and the retinal 

 layer the vitreous body of the eye forms. The later stages of 

 the development of the eye take place after the embryo leaves 

 its capsule. The nerve-fibres that develop from the retina 

 and pass into the brain along the optic stalks have not yet 

 appeared. 



The Ears 



While the neural groove is closing, a pair of thickened circu- 

 lar patches of the inner layer of the ectoderm arises, one on 

 each side of the head near the hind-brain. After the closure 

 of the neural tube each area forms a shallow pit with the con- 

 cavity turned outward, and each is covered by the outer layer 

 of the ectoderm. The pit deepens, the outer edges come 

 together, and a hollow vesicle is formed before the tadpole 



Fie. 49. — Cross-section through head and 

 eyes. F. Fore-brain. L. Lens of eye. 

 OP. Optic cup. OS. Optic stalk. PH, 

 Pharynx. PT. Pituitary body. ST. Sto- 

 modaeum. 



