412 NORMAL HISTOLOGY AND ORGANOGRAPHY. 



this fissure and fill the cavity of the secondary vesicle. 



Fig. 283. Part of a section through the head of an early human 

 embryo, showing the connection of the primary optic vesicles with the 

 fore-brain (His): olf, Olfactory area of epiblast; ch., part of fore-brain 

 which gives rise to cerebral hemispheres; th, thalamencephalon; p.o.v., 

 primary optic vesicles. 



Fig. 284. Three successive stages of development of the eye, showing 

 formation of secondary optic cup and crystalline lens in human embryos 

 of 4 mm. (A), 6 mm. (#), and 8 mm. (C) (Tourneux): a, a, primitive 

 optic vesicles; b, external layer of secondary optic cup (future pigment 

 layer of retina) ; c, inner layer of cup (retina proper) ; d, lens pit (thick- 

 ened and depressed ectoderm) ; e, lens vesicle. 



These cells form the vitreous humor, while the choroid 



