494 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



Fig. 163. — Diagrammatic, indicating 

 course of lens fibers and in heavier lines 

 the two sutures in which the ends of the 

 fibers meet (after Rabl, i 



icle may be merely a differential activation, decreasing radially from the 

 region most affected. 



The lens fibers extend between the two polar regions, forming concen- 

 tric layers; at each pole they come together along a line, forming a short 



suture (Fig. 163). In the amphibian 

 eye the inner suture, toward the 

 retina, is anteroposterior, the eye be- 

 ing regarded as lateral; the outer, 

 dorsiventral in direction. If presump- 

 tive lens epidermis, optic plate, or 

 early optic vesicle is turned 90°, it 

 is possible, up to a certain stage, to 

 obtain lenses normal in relation of 

 sutures to the optic cup; but during 

 closure of the neural folds, that is, 

 before the lens develops at all, the 

 suture pattern is fixed and not al- 

 tered by turning (Woerdeman, 1934). 

 Apparently, then, although it may 

 be altered by an inductor up to a 

 certain stage, it is determined in relation to the general pattern quite in- 

 dependently of the inductor, perhaps in relation to dorsiventral or antero- 

 posterior gradient pattern or both. 



LENS DEVELOPMENT IN OTHER VERTEBRATES 



Lenses appear frequently, either in the normal location or in other 

 head regions without relation to eyes or contact with nervous tissue, in 

 teratological embryos of teleost fishes, both those occurring occasionally 

 in nature and those experimentally produced. They may even develop 

 in anophthalmic forms, and several supernumerary lenses may be present 

 in an individual ; they vary in size and degree of development but may 

 be large and advanced in differentiation.-^' It seems beyond question that 

 some of these lenses develop from other than presumptive lens epidermis. 

 The factors concerned in their origin are not known, but their develop- 

 ment suggests a condition in the epidermis associated with stage of de- 

 velopment and region of body rather than with action of a specific induc- 

 tor. According to Werber, however, blastolysis, that is, a dissociation of 

 parts of organ primordia, occurred in consequence of exposure to inhibit- 

 « Mencl, 1903, 1908; Gemmill, 1906a, b; Stockard, 1909, 19106, c; Werber, 1916a, b, 1918. 



