810 



SCIENCE 



[N. S. Vol. XZXVI. No. 937 



it capable of producing lenses — is in no 

 way determined by the action of the optic 

 vesicle. The effect proceeding from the 

 optic vesicle serves merely as the trigger to 

 set off the separate mechanisms of the 

 superficial cells. We may conceive the 

 cells, then, to be absolutely independent of 

 one another in the matter of lens forma- 

 tion. Their concerted action is the purely 

 accidental result of the fact that they suf- 

 fered simultaneously a change in their en- 

 vironment, that is, the effect derived from 

 the optic cup. This effect merely initiates 

 the development of the lens. Neither the 

 ectodermal organization which causes that 

 development nor the process of develop- 

 ment is determined by the optic vesicle. 

 Even if lens development required the con- 

 tinuous action of an effect from the optic 

 vesicle, this view of the relation need in no 

 wise be altered, for that continuous action 

 would constitute merely a persistent fea- 

 ture of the environment appropriate to the 

 operation of the separate mechanisms of 

 the ectodermal cells. It is possible, as 

 some experimental data seem to indicate, 

 that regions of ectoderm remote from those 

 which normally give rise to lenses are ca- 

 pable of producing lenses as a result of the 

 action of transplanted optic vesicles.^ If 

 this is true, the fact would seem to put con- 

 siderable strain upon the view just out- 

 lined. Nevertheless, it is always possible 

 to buttress up a favorite hypothesis with 

 subsidiary hypotheses. If the main thesis 

 is highly esteemed, often some very com- 

 plicated accessory hypotheses will be tol- 

 erated. I am sure that any such diffi- 

 culty as the present one — and the experi- 

 mental work upon embryos has yielded 



- Lewis, W. H., 1904, ' ' Experimental Studies 

 on the Development of the Eye in Amphibia," 

 American Journal of Anatomy, Vol. 3, No. 4, 

 pp. 505-536. See also later papers by the same 

 author. 



many such — will readily yield to this treat- 

 ment. I wiU leave the task for those to 

 whom this conception of organization is 

 the favorite one. 



"What other interpretation can be put 

 upon this matter of lens formation? The 

 essential feature of the process is the con- 

 certed action of ectoderm cells. We may 

 regard this concerted action as due to an 

 agent which immediately exercises general 

 control over the behavior of all the cells 

 concerned. If it is true that the optic ves- 

 icle has something to do with the invagina- 

 tion of the lens, it is conceivable that the 

 substance of the optic vesicle is a seat of 

 energy which is somehow brought to bear 

 upon the near superficial ectoderm, with 

 the result that its cells are compelled to 

 execute those changes of form and relative 

 position which are involved in the shaping 

 of a lens. We should have to attribute to 

 the ectoderm cells similarity of structure 

 and an inherent mechanism sufficient to 

 render them capable of responding to the 

 control of the optic vesicle. The expres- 

 sion "concerted action of ectoderm cells" 

 should not convey the impression that 

 every cell behaves precisely like every 

 other. Obviously such can not be the case. 

 The lens invagination is not exactly hemi- 

 spherical. The changes in form and posi- 

 tion of the cells must vary according as 

 whether the cells come to lie nearer the 

 axis or nearer the periphery of the invagi- 

 nation. Upon the first view which we have 

 outlined, the factors which determine the 

 differences in the behavior of the individ- 

 ual cells are contained within the mechan- 

 isms of the independently acting cells 

 themselves. Upon the second view, which 

 we are now presenting, the differential fac- 

 tors of lens formation lie outside the group 

 of lens cells. So far as internal conditions 

 are concerned, those cells may be precisely 

 alike. 



