THE EYE IN AMPHIBIA. 247 



evidence of this similitude is, as I think, fully demonstrated by 

 the figures 9, 10 and u in my paper. Again, I have by no 

 means considered the anlage of the lens within the ectoderm as 

 a regular (i. c., functional) sense organ (" Hautsinnesorgan "), 

 as I have especially emphasized in the statement : Es " liegt uns 

 selbstverstandlich der Gedanke fern, unsere Lentoide functionell 

 auf gleiche Stufe mit einer Sinnesknopse stellen zu wollen, . . .". 

 I have always spoken of a phylogenetic, never of an ontogenetic 

 deduction of the lens from a primitive sense organ. Finally I 

 have expressed the opinion that the formation of " Sinnesknos- 

 pen " is one of the most primitive tendencies of all ectodermal 

 elements and that in any part of the ectoderm a bud-organ may 

 arise under adequate stimulus. This being admitted, the conclu- 

 sion seems to me justified that also the contact stimulus of the 

 optic vesicle upon the ectoderm may awaken these most primi- 

 tive properties of the ectodermal cells and result in the formation 

 of a structure morplwlogically homologous with a primitive sense 

 organ which in the course of phylogenetic development has gradu- 

 ally entered a new path of differentiation. 



Taking all this into consideration, the absence of preformed 

 primitive sense organs in t/iat part of the skin where the optic ves- 

 icle comes in contact u>it/i it and where in consequence the lens is 

 formed is by no means an objection against my theory. When my 

 theory holds that the lens, from a morphological point of view, 

 has to be considered as an ancient sense organ, which arose, it is 

 true, originally from a primitive sense organ of the skin ("Placo- 

 den-Organ " of Kupffer], it does not of course imply the necessity 

 that during ontogenetic development of later animal forms we 

 should still be able to see the lens evolve directly from such an 

 organ. Just as the mode of development of so many other 

 organs, that of the lens also may have undergone, in the course 

 of phylogenetic progress, considerable modifications until the 

 original process of differentiation becomes more or less concealed. 

 Now, if an organ, as in the case of the lens, still exhibits in its 

 early ontogenetic features some distinct reminiscences of ancestral 

 characteristics, we are all the more justified in drawing conclusions 

 from such phenomenon in regard to the ancestry of the organ in 

 question. 



