64 



Heymons resultater, at midttanucn hos Myriapoderne dan- 

 nes fra entodermen og hos visse insekter fra ektodermen 

 som en sjelden uoverensstemmelse. 



Det s_ynes, som om man heraf maa kunne drage den 

 slutning, at udviklingens forloh ikke altid forst og fremst 

 betinges af slægtskabet (den fylogenetiske udvikling). Der- 

 imod synes udviklingen i høiei'e grad at være modificerbar 

 end man vel hidtil almindelig har antaget. Anlærjgets, he- 

 gyndehesstadiets form og bygning synes at spille en stor 

 rolle, ja synes' altsaa endog at kunne bringe enhver saakaldot 

 rekajjitulation til at bortfalde, et forhold, som vel er det 

 samme som det af Haeckel kaklet caeitoyeni. Interessant 

 er i denne henseende, at d? forskjellige Ascidiers knop- 

 skydninger indbyrdes viser ligesaa store overensstemmelser 

 som embryonaludviklingerne, skjønt knopskydningerne visse- 

 lig intet har med rekapitulation at gjøre. 



Et eksperimentalt bevis for udviklingens store foran- 

 derlighed er givet af G. Wolif (Entwicklungsphysiologisehe 

 Studien. Archiw fiir Entwicklungsmekanik. Bd. I. 1895), 

 der fandt, at naar linsen hos Tri ton taeniatus fjernedes, 

 regeneredes den fra epithelcellerne i Iris. Her er altsaa 

 ogsaa en nydannelse, om ikke af et helt individ, saa dog 

 af et organ og ogsaa denne nydannelse finder sted paa en 

 væsentlig anden maade end under embryonaludviklingen. 

 Kan imidlertid saa store forandringer i den maade, hvor- 

 l)aa nydannelser tinder sted. forekomme, saa synes det mig 

 at vise, at man endnu maa udtale sig med stor forsigtig- 

 hed om, hvad der er fylogenetisk rekapitulation. 



Man maa fremdeles undlade at drage altfor vidtgaa- 

 ende slutninger fra udviklingshistorien over de forskjellige 

 typers slægtskabsforhold. Naar enkelte forfattere gaar saa 

 vidt, at de siger, at de tilsvarende organer hos Ascidie- 

 knopperne og Ascidielarven ikke er „homologe'', fordi de ikke 

 dannes paa tilsvarende maade af kimUadene, da viser dette 

 tilstrækkelig, til hvilken forvirring en ensidig vurdering af 

 udviklingshistorien kan fore. Thi hvis de tilsvarende orga- 

 ner, som centralnervesystemet, ikke er homologe hos indi- 

 vider af samme art, da er vel dermed overhovedet enhver 

 morphologisk betragtning, end sige da sammenlignende be- 

 tragtning opgiven. 



For man udnytter udviklingshistorien, især dens erfa- 

 ringer fra de tidligere stadier til indgaaende sammenligning 

 mellem de forskjellige typer, maa man utvivlsomt søge at 

 Jdargjøre sig de ahnindcligc lore for udviklingen i Iwiere 

 grad end hidtil. Og under dette arbeide vil utvivlsomt 

 studiet af kimbladene, deres dannelse og skjæbne forblive 

 et af den niorpliologigke udviklingsliistories vigtigste pro- 

 blemer. 



Jena, Januar 1896. 



develoi)ment. In this respect, Heymons conclusion, that 

 the mid-gut in Myriapoda is formed from the entoderm, 

 and in certain insects from the ectoderm, stands as an 

 exceptional incongruity. 



From this it seems as if one ought to be able dra w 

 the conclusion that the course of develo]>ment is not always 

 primarily contingent upou relationshii) (the phylogenetic 

 development). On the other hand the development seems 

 to be more capable of moditication than has hitherto ge- 

 nerally been supposed. The form and structure of the 

 rudiiiieiit, the primitive stage, seems to play an important 

 part, to be able, indeed. to cause the abandoning of all 

 so called recapitulation, a circamst;ince which. is probably 

 the same as Haeckers so-called caeiiogeiiy. It is interesting 

 to tind that in this respect tho buddings in the different 

 Ascidian groups exhiljit just as great conformity to one 

 another, as do the embryonic developments, though the 

 buddings certainly have nothing to do with recapitulations 



One experimental proof of the great variableness of 

 development is given by G. Wolft' (Entwicklungsphysiolo- 

 gische Studien. Archiv fiir Entwicklungsmechanik. Bd. 

 I. 1895), who found that when the lense in Triton taeniatus 

 was removed, it was regenerated from the epithelial cells 

 in the iris. Thus here too there is a new formation, if 

 not of an entire animal, yet of an organ, and this new 

 formation, too, takes place, in a manner essentially different 

 to tliat in embryonic development. If. however, such great 

 changes can occur in the manner in which the new forma- 

 tion takes place, it seems to me to sliow that great caution 

 must still be used in assertions as to what is phylogenetic 

 recapitulation. 



It will not do, either, to draw too far-going conclu- 

 sions from the developmental history of the relationship of 

 the various types. Wlien certain writers go so far as to 

 say that the corresponding organs in the Ascidian bud and 

 the Ascidian larva are not ..homologous", because they are 

 not formed in a corresponding manner from the germ- 

 layers, this sufficiently shows to what confusion a one-sided 

 valuation of developmental history may lead ; for if corre- 

 sponding organs, such as the central organs of the nervous 

 system, are not homologous in animals of the same species, 

 then every niorphological view, to say nothing of compara- 

 tive views, must be given up. 



Before utilising developmental history, and especially 

 its experiences of tho earlier stages, for a detailed com- 

 parison between the different types, we mast endeavour to 

 clear up the general laws of developuient more completely 

 tJian heretofore: and in doing this, the study of germ-layers, 

 their formation and destiny will undoubtedly continue to 

 be one of the greatest problems of morpJioJogical develop- 

 mental history. 



Jena. January, 1896. 



