104 FRANKLIN PEARCE REAGAN 



It is evident that the development of the otic capsule and 

 of the stapedial plate belongs in that class of phenomena which 

 have been designated as the 'interactions of parts' and again as 

 'dependent development.' Hertwig ('94) was one of the first 

 to attach great importance to those developmental process which 

 are due only indirectly- to the original constitution of the fer- 

 tilized ovum itself and likewise those due only indirectly to the 

 external environment. These he designated as the "perpetu- 

 ally changing mutual relations in which cells of an organism are 

 placed to one another." According to Herbst, all movements, 

 tropic or tactic, and many processes of differentiation are re- 

 sponses of a formative, as well as a directive nature. Thus far 

 the clearly demonstrated cases of interaction of parts are rela- 

 tively few. Loeb has shown that the position occupied by the 

 pigment cells on the yolk-sac of Fundulus is an oxygenotatic 

 reaction. Several cases have been reported in which early dis- 

 placed embryonic cells have resumed their original position. 



The development of certain parts of the vertebrate eye has 

 been shown to be of a 'dependent' sort; there is, however, a 

 certain amount of disagreement over the experimental results 

 obtained by various observers. Spemann showed that if the 

 formation of the optic vesicle of the frog be inhibited by injury 

 to the medullary plate, or that if its approximation to the ecto- 

 derm be prevented, a lens will not form. Lewis obtained simi- 

 lar results on Ambly stoma. He showed that ectoderm from other 

 regions transplanted to that region approximated by the optic 

 vesicles would give rise to a lens. Schaper removed the entire 

 central nervous system from a tadpole except the fore-brain and 

 optic vesicles. He found that a lens developed in situ in the 

 ectoderm and not as an infolding from it; he believed the lens 

 was self-differentiating. On the other hand. King found that 

 lenses might develop in the entire absence of an optic cup in the 

 tadpole, while Mencl found the same to be true in an abnormal 

 embryo of Salmo salar. In these two latter cases it is possible 

 that stimuli exerted by the fore-brain itself caused lens-forma- 



2 Clark ('15j wrongly regards such phenomena as those in which hereditary 

 constitution plays no part. 



