Warren Harmon Lewis 533 



some kind between the two tissues, by means of which we can readily 

 imagine that an interchange of protoplasm or of certain substances in 

 the protoplasm may take place. With this interchange there may take 

 place a definite chemical reaction producing within the ectodermal cells 

 new substances which give to them new properties, that of forming a 

 lens being the predominant one. That no new substances flowing from 

 the ectoderm into the optic vesicle cells are necessary for their subse- 

 quent differentiation will be shown in another article. 



The contact of the optic vesicle and ectodermal cells without inter- 

 change of substances may, on the other hand, produce the changes neces- 

 sary for the lens formation. In whatever manner the change is brought 

 about, it would seem evident that chemical changes have taken place 

 within these lens-forming ectodermal cells, and that the optic vesicle 

 cells by some means have been able to change the chemical nature of 

 these cells, new substances being formed which are now peculiar to the 

 groups of cells that show such a modified development. It would seem 

 probable that the modified development was directly due to these new 

 chemical substances. 



It is not unlikely that a new biological chemistry must be developed, 

 its reactions being those between living tissues or between a living tissue 

 and the product of a living tissue. In the example of the lens reaction 

 it seems evident that living ectodermal cells must be acted upon, but it 

 is not so evident that the living optic vesicle cells are necessary, as it 

 may be possible to extract from them substances which will give the 

 lens reaction. By means of such a biological chemistry some of the 

 properties of the various tissues and organs may be discovered and per- 

 haps a deeper insight into the problems of development obtained. The 

 development of a biological chemistry is to be considered as but one of 

 the phases of correlative embryology and in considering the correlations 

 in development the mechanical factors must not be neglected. 



There are many more questions in connection with lens formation 

 which suggest themselves. For how long a period does the ectoderm 

 possess the lens-forming property? Are there progressive changes in 

 the ectodermal cells which give to them at the proper time, and only at 

 this period, the power to respond to the stimulus of the optic vesicle? 

 It would seem an easy thing to determine in part at least this question. 

 It seems probable that the epithelial cells of the otic capsule undergo 

 such differentiation as to prevent their response to a stimulus from the 

 optic vesicle leading to lens formation. The several instances in which 

 the transplanted optic vesicle is in direct contact with the otic capsule 

 cells without indication of lens formation would seem to suggest this 



