110 



THE GENESIS OF THE VERTEBRATE EYE 



fibers which were formed directly from the cells of the posterior wall of 

 the lens vesicle. The further conversion of crop after crop of cuboidal 

 epithelial cells, at the equator of the lens, results in layer after layer of 

 fibers each of which is added outside of the previous one (Fig. 41b). 

 Any one fiber being too short to stretch from pole to pole of the lens, 

 its anterior and posterior ends meet, head-on, the corresponding ends 



Fig. 40 — Early stages of the lens. Redrawn from Mann. 



a, placode stage, comparable with Figure 38a'; p- lens placode formed in surface ectoderm; 

 ov- optic vesicle, b, c, lens pit forming and closing off; cl- lips of optic cup. d, lens vesicle 

 has detached and passed into mouth of optic cup; Iv- lens vesicle; m- mesenchyme which has 

 now invaded space between optic cup and surface ertoderm. e, cavity of lens vesicle being 

 obliterated by the elongation of the posterior wall cells to form the first of the lens fibers. 

 f, lens is now solid, and its present fiber mass will constitute the 'embryonic nucleus' of 

 later life (cf. Fig. 41b); ^ zone of transition of epithelium into fibers — the locus at which 

 all future fibers will form; ac- anterior chamber forming as a cleft in the mesoderm, separ- 

 ating the latter into the future cornea and the future iris stroma, g, new fibers have been 

 added to the embryonic nucleus and are meeting end-to-end at anterior and posterior suture 

 planes; s- posterior suture {cf. Fig. 41b, sp). 



of a diametrically opposite fiber. These meeting points are aligned in 

 radial planes within the lens mass, called lens sutures (Figs. 40g, 41b 

 and c) , which perforce branch more and more toward the surface of the 

 growing lens as the number of epithelial cells ringing the equator in- 

 creases and the number of fibers seeking place for their tips against the 

 suture planes increases. At any one time, there are many superficial 

 layers in which the fibers have not yet elongated enough to reach suture 



