98 EARLY EMBRYOLOGY OF THE CHICK 



tween the sensory and the pigment layers of the retina (Fig. 

 36, B). Later when these two layers fuse this space is entirely 

 obliterated. 



While the secondary optic vesicles are usually spoken of as 

 the optic cups, they are not complete cups. The invagination 

 which gives rise to the secondary optic vesicles, instead of be- 

 ginning at the most lateral point in the primary optic vesicles, 

 begins at a point somewhat toward their ventral surface and is 

 directed mesiodorsad. As a result the optic cups are formed 

 without any lip on their ventral aspect. They may be Hkened 

 to cups with a segment broken out of one side. This gap in 

 the optic cup is the choroid fissure (Fig. 35). In Figure 36, B, 

 a section is shown which passes through the head of the embryo 

 on a slight slant so that the right optic cup, being cut to one 

 side of the choroid fissure appears complete while the left optic 

 cup being cut in the region of the fissure shows no ventral lip. 



The infolding process by which the optic cups are formed 

 from the primary optic vesicles is continued to the region of 

 the optic stalks. As a result the optic stalks are infolded so 

 that their ventral surfaces become grooved. Later in develop- 

 ment the optic nerves and blood vessels come to He in the 

 grooves thus formed in the optic stalks. 



The Lens. — The lens of the eye arises independently of the 

 optic vesicles, from the superficial ectoderm of the head. The 

 first indications of lens formation appear in chicks of about 

 40 hours as local thickenings of the ectoderm immediately over- 

 lying the optic vesicles. These placodes of thickened ectoderm 

 sink below the general level of the surface of the head to form 

 small vesicles which extend into the secondary optic vesicles. 

 Their opening to the surface is rapidly constricted and even- 

 tually they are disconnected altogether from the superficial 

 ectoderm. At this stage the opening to the outside still persists 

 although it is very small (Fig. 36, B, right eye). In sections 

 which do not pass directly through the opening, the lens vesi- 

 cle appears completely separated from the overlying ectoderm 

 (Fig. 36, B, left eye). 



The derivation of the lens from a placode of thickened epi- 

 thelium which sinks below the general surface, and eventually 

 loses its connection with the superficial ectoderm, is strikingly 

 similar to the early steps in the derivation of the auditory 



