OPPEL. KEIBEL 349 



higher animals ; young stages are more alike than old 

 stages. . . . The differences which these similar series show 

 (for which reason they cannot be regarded as identical) may 

 be designated as temporal disturbances in the degree of 

 development of the separate organs or organ-systems. Some 

 organs show very considerable temporal dislocations, others 

 a moderate amount, others again an inconsiderable amount. 

 Among the developmental stages of various higher animals 

 can be found some which correspond to the ancestral forms 

 and also to the lower types which resemble these ancestral 

 forms. On the basis of the tabulated data here given there 

 can be distinguished with certainty in the ontogeny of 

 Amniotes a pro-fish stage, a fish-stage, a land-animal stage, 

 a pro-amniote stage, and following on these a fully developed 

 reptile, bird or mammal stage." 1 



Oppel's methods were employed by Keibel 2 in his 

 investigations on the development of the pig, which formed 

 the model for the well-known series of Normentafdn of the 

 ontogeny of Vertebrates which were issued in later years 

 under Keibel's editorship. Keibel was more critical of the 

 biogenetic law than Oppel, and he held that the ancestral 

 stages distinguished by Oppel could not be satisfactorily 

 established. He suggested an interesting explanation of 

 heterochrony in development, according to which the 

 premature or retarded appearance of organs in ontogeny 

 stands in close relation with the time of their entering upon 

 functional activity. Thus in many mammals the mesodermal 

 part of the allantois often appears long before the endodermal 

 part, though this is phylogenetically older. This Keibel 

 ascribes to the fact that the endodermal part is almost 

 functionless. " One can directly affirm," he writes, " that the 

 time of appearance of an organ depends in an eminent 

 degree upon the time when it has to enter upon functional 



1 Quoted by Keibel, Ergebn. Anat. Entwick., vii., p. 741. 



2 "Studien zur Entwickelungsgeschichte des Schweines," Schwalbe's 

 Morphol. Arbeiten, iii., 1893, and v., 1895. 



Normentafeln zur Entwickelungsgeschichte des Sc/i?vetncs,]e-na., 1897. 



" Das biogenetische Grundgesetz und die Cenogenese," Ergebn. 

 Anat. Entw., vii., pp. 722-92, 1897. 



" U. d. Entwickelungsgrad der Organe," Handb. vergl. exper. 

 Enhvick. der Wirbelthiere, iii., 3, pp. 131-48, 1906. 



