6a 2 Dritte Sitzung der fünften und zehnten Sektion. 



originating lenses arise from regions of ectoderm that are not in 

 contact with either optic vesicle, the brain wall, or any nervous or 

 sensory organ of the individual. 



The lens-bud is capable of perfect self-differentiation. No 

 contact at any time with an optic vesicle or cup is necessary. These 

 lenses finally become typical transparent refractive bodies exactly 

 similar in histological structure to a lens in the normal eye. 



The size and shape of the lens is nor entirely controlled by the 

 associated optic cup. Lenses may be abnormally small for the 

 size of the cup, or entirely too large, so that they protrude ; or, 

 finally, peculiarly shaped oval or centrally constricted lenses may 

 occur in more or less ordinarily shaped optic cups. The lens is by 

 no means always adjusted to the structure of the optic cup as has 

 been claimed by some observers. 



An optic vesicle or cup is invariably capable at some stage of 

 its development of stimulating the formation of a lens from the 

 ectoderm with which it comes in contact. It is remarkable how 

 extremely small an amount of optic tissue is capable of stimulating 

 lens formation from the ectoderm. 



The optic vesicle may stimulate a lens to rom from regions 

 of the ectoderm other than that which usually forms a lens. This 

 is shown by the fact that a median cyclo'pean eye always stimulates 

 a lens to form from the overlying ectoderm. It is scarcely possible 

 that the lateral normal lens-forming ectoderm could follow 

 the cyclopean optic cup to the many stränge situations it finally 

 reaches. 



The ectoderm of the head region is more disposed to the 

 formation of lenses than that of other parts of the body, since free 

 lenses invariably occur in this region. 



A deeply buried optic vesicle or cup may fail to come in con- 

 tact wit the ectoderm*; in such cases it lacks a lens. The tissues of 

 the embryonic cup in the fish are unable to form or regenerate a 

 lens. This is not true for all embryos as has been shown for one 

 species of frog. 



The optic lens may be looked upon as a once independent organ 

 (possibly sensory or perhaps an organ for focusing light on the 

 brain wall, before the vertebrate eye had arisen) which has become 

 so closely associated with the nervous elements of the eye that it 

 has to some extent lost its tendancy to arise independently, al- 

 though still capable of doing so under certain conditions. The lens 

 now arises much more readily in response to a Stimulus from the 

 optic vesicle, a correlated adjustment which insures the almost 

 perfect normal accord between the optic cup and the lens. 



Finally it may be stated in brief, that the optic vesicle or cup 

 always has the power to stimulate a lens to arise from any ectoderm 

 with which it may come in contact during certain stages of its 



