ORIGIN OF THE LENS 133 



then at least, and perhaps forever, the evolutionary origin of the verte- 

 brate lens must remain a tantalizing mystery. 



A very good question is: how is it that the lens, derived from the 

 skin, lies inside the fibrous and uveal tunics— which, above, we homolo- 

 gjzed with the meningeal coats of the brain? Did the retina acquire its 

 optical partner before the central nervous system acquired its protective 

 sheaths? Perhaps so — and, such theories as that of Tretjakoff make such 

 an assumption necessary. But the lens could easily enough have gotten 

 through the sclerotic coat after the latter had evolved. Such legerdemain 

 is common enough in vertebrate history^as witness the presence of the 

 pectoral girdle inside the rib basket, in the turtles. All that is needed is 

 a nice timing of embryological events, occurring as an embryonic muta- 

 tion — if the lens did pass through the dura mater to get inside the eye- 

 ball, ii assuredly did so in one jump, in some ancient embryo in which 

 the condensation of the dura happened to be delayed. And lenses have 

 been getting inside of eyes ontogenetically in that same way ever since. 



