DYNAMICS OF REGENERATIVE PROCESSES 215 



ceded by the formation of the optic vesicles, and where the latter 

 touch the ectoderm a proliferation of cells begins, from which later the 

 lens is formed. Spemann* has shown in the salamander that if the 

 optic vesicle does not reach the ectoderm, no lens is formed. The same 

 author showed, moreover, that if in the embryo the optic vesicle is 

 destroyed, no lens is formed, but that if afterward the optic vesicle is 

 regenerated, a lens is formed as soon at this vesicle touches the ecto- 

 derm. Lewis confirmed and enlarged Spemann's observations.f 



He showed" that the ectoderm in frogs can form a lens at any place 

 in the body if the optic vesicle is transplanted and allowed to come in 

 contact with the ectoderm. He suggests that definite chemical reac- 

 tions may occur between certain substances of the optic vesicles and the 

 cells of the ectoderm; that these substances determine the formation 

 and the peculiar character of the cells of the lens. This suggestion is 

 in harmony with the ideas of Sachs, and I am inchned to believe that 

 it will lead to further discoveries. Lewis's experiments also throw light 

 upon an earlier observation made on salamanders. It was found that 

 if a lens is removed in a salamander it can be regenerated from the 

 iris. Inasmuch as the edge of the iris is naturally in contact with the 

 retina (optic vesicle), this is about what should be expected from Lewis's 

 experiments. 



It is well known that the skeletal muscles degenerate if separated 

 from the centers of their motor nerves. Goltz and Ewald have cut 

 long pieces from the spinal cord in dogs, and observed that all the mus- 

 cles belonging to the excised segments of the spinal cord degenerated, as 

 was to be expected. This may be due to some chemical change in the 

 muscles, owing to their inactivity after the motor nerves are cut pr sepa- 

 rated from their ganglia. 



We have already discussed one case of an apparent action at a dis- 

 tance in Tubularians, where the suppression of the process of regener- 

 ation at the oral pole accelerated the formation of a polyp at the aboral 

 pole. Przibram found a case of distance action which is not so easy 

 to explain. J In a Crustacean, Alpheus, the right and left chelas are 

 hot equal in size and form. The same type of chela is not always on 

 the same side, but in about one half of the cases the one type is on 

 the right side, in the other half, the other type. Przibram found that 

 if the larger of these chelae is removed in such a Crustacean, the remain- 

 ing chela assumes after the next moulting the shape of the removed 

 chela, and the regenerating chela assumes the shape which the remain- 



* Spemann, Siizungsber. der phydk. med. Geselkchafl in Wunburg, 1901. 



t Lewis, Am. Jour. Anatomji, Vol. 3, 1904. 



J H. Przibram, Archiv fur Entuiickelungsmechartik, Vol. 11, p. 329, 1901. 



