1.1 ORTHOGENESIS 75 



moment, right before our eyes, nature arrives at identical 

 results, in sometimes neighboring species, by entirely 

 different embryogenic processes. Observations of "heter- 

 oblastia" have multiplied in late years, 1 and it has been 

 necessary to reject the almost classical theory of the 

 specificity of embryonic gills. Still keeping to our compari- 

 son between the eye of vertebrates and that of molluscs, 

 we may point out that the retina of the vertebrate is 

 produced by an expansion in the rudimentary brain of 

 the young embryo. It is a regular nervous centre which 

 has moved toward the periphery. In the mollusc, on the 

 contrary, the retina is derived from the ectoderm directly, 

 and not indirectly by means of the embryonic encephalon. 

 Quite different, therefore, are the evolutionary processes 

 which lead, in man and in the Pecten, to the development 

 of a like retina. But, without going so far as to compare 

 two organisms so distant from each other, we might reach 

 the same conclusion simply by looking at certain very 

 curious facts of regeneration in one and the same organism. 

 If the crystalline lens of a Triton be removed, it is re- 

 generated by the iris.* Now, the original lens was built 

 out of the ectoderm, while the iris is of mesodermic origin. 

 What is more, in the Salamandra metadata, if the lens be 

 removed and the iris left, the regeneration of the lens takes 

 place at the upper part of the iris; but if this upper part 

 of the iris itself be taken away, the regeneration takes 

 place in the inner or retinal layer of the remaining region.* 



1 Salensky, ' Heteroblastie " (Proc. of the Fourth International Con- 

 gress of Zoology, London, 1899, pp. 111-118). Salensky has coined 

 this word to designate the cases in which organs that are equivalent, 

 but of different embryological origin, are formed at the same points 

 in animals related to each other. 



1 Wolff, "Die Regeneration der Urodelenlinse" (Arch. f. Entwick- 

 elungsmechanik, i., 1895, pp. 380 ff.). 



8 Fischel, "Uber die Regeneration der Linse" (Anat. Anzeiger, xiv., 

 1898, pp. 373-380). 



