50 



Der reiser sig imidlertid da de sporgsmaal : Hvorledes 

 forholder sig en knopskydniiig ud fra et saa primitivt an- 

 læg til kimbladlæren. og hvorledes stemmer saa den senere 

 knopskydiiings forlob med læren om rekapitulation i udvik- 

 lingen. med den fylogenetiske grundlov. I de følgende ka- 

 ])itler vil jeg i al kortlied redegjøre for, hvad de lier vundne 

 resultater synes at vise i denne retning. 



Kapitel 5. 

 Ascidienies kiiopskyduiiig og kimbladlæren. 



Den forste kimldadlære var den erfaringsmæssige 

 vundne erkjendelse. at dannelsen af et bestemt organ over- 

 alt i dyreriget eller iallefald inden storre grupper sker ud 

 fra det samme „priniære kimblad" ; saaledes fandt man, at 

 eentralnervesystemet altid dannedes af ektodermen. JJed 

 ,.primære kimblade- har da utvivlsomt de fleste zoologer 

 forstaaet de l)ladformige epithelmembraner. der tidlig i ud- 

 viklingen dannes ved den som gastrulation beskrevne udvik- 

 liugsproces. 



I de fleste gru])i)er i dyreriget bar man kunnet be- 

 skrive et tobladet ,.gastrulastadium", og om dette end i de 

 forskjellige grupper liar vist sig at fremvise forskjelligbeder, 

 saa har man dog ment. at gastrulationen overalt var en 

 saa tilsvarende proces, at den overalt maatte antages at 

 betegne en bestemt udviklingsepoche. Saaledes siger brø- 

 drene Hertwig {2{S. pag. 2): ..Entwicklungsgeschichtliche 

 Forschungen, welclie im Anschluss an die Gastraeatheorie 

 die tibrigen Stadien der Thierentwicklung auf demselben 

 Wege erklareu vollen, werden naturgemass von der Frage 

 anszugehen liaben. in welcher Weise entwickelt sich der zwei- 

 bliittrige Keim zu bciherer Organisatioii." Dette, at for- 

 følge de to gastrulablades senere skjæbne under udviklingen 

 og specielt at undersoge, hvilke organer hvert blad danner 

 har utvivlsomt været det morphologiske problem, som de 

 fleste zoologer har opfattet som det egentlige udviklings- 

 historiske kimbladspørgsmaal. I de store hovedtræk kan 

 man ogsaa sige, at svaret har været det, at de samme, 

 ,,ligeværdige" organer som f. eks. eentralnervesystemet over- 

 alt dannes i)aa en meget overensstemmende maade fra kim- 

 bladeue. INIan vil derfor utvivlsomt til alle tider erkjende, 

 at der med kimbladlæren har været udtalt en vigtig lov- 

 mæssighed, og at denne lære i hoi grad har fort embryo- 

 logien fremad. 



Der har dog helt siden Balfunrs tid reist sig mange 

 indvendinger mod kimbladlæren. Man har hævdet, at denne 

 ikke gjæilder med nogen absolut lovmæssigbed. Saaledes 

 har nogle forskere paavist, at gastrulastadiet ikke overalt 



The following (j^uestions. however, will arise: In what 

 relation does budding from so primitive a rudiment stand 

 to the germ-layer theory: and how does the subsequent 

 progress of budding harmonise with the theori of recapi- 

 tulation in the development, — ■\vith the phylogenetic fun- 

 damental Lnv? In the foUowing chapters, I will briefly 

 state what the results here arrived at seem to prove in 

 that direction. 



Cliapter V. 

 Gemmation in Asfidians and tlie (ierni-layer Tlieory. 



The earliest germ-layer theory was the empirically 

 gained acknowledgt'ment that the formation of a certain 

 organ everywhere in the animal kingdom, or at any rate 

 within the larger groups, originates in the same ,,priniary 

 germ-layer." The central organs of the nervous system were 

 thus considered to be ahvays formed from the ectoderm. 

 By ..primary germ-layer-'. most zoologists have undoubtedly 

 understood the laminar, epithelial membranes, wbich are 

 lormed early in the development, by the process described 

 as gastrulation. 



In most groups of the animal kingdom, it has been 

 possible to describe a bi-laminar ..gastrula stage"; and even 

 if this has proved to exhibit ditferences in the various 

 grou])S. it has still been thought that gastrulatiim was a 

 so universally similar process, that it might always be as- 

 sumed to indicate a certain developraental epoch. The 

 brothers JJcrtii-iij write as follows (26, p. 2): ..Eutwicklungs- 

 geschichtliche Forschungen, welche im Anschluss an die 

 Gastræatheorie die iibrigen Stadien der Thierentwicklung 

 auf demselben Wege erklaren wollen, werden naturgemass 

 von der Frage anszugehen habeu, in welcher Weise entwic- 

 kelt sich der zweiblattrige Keim zu hoherer Oi'ganisation." 

 To folio w the subsequent fate of the gastrula lavers during 

 development, and in particular to investigate the question 

 as to wbich organs each layer forms, has undoubtedly been 

 the morphological problem -which most zoologists have re- 

 garded as the true developmental-historical germ-layer 

 question. In its main features too, it may be said that 

 the answer has been, that the same equally important or- 

 gans as, for instance, the central organs of the nervous 

 system, are formed everywhere in a very similar mannei- 

 from the germ-layers. It will tliercfore undoubtedly always 

 be acknowledged that in the germ-layer theory an impor- 

 tant conformity to law has been stated, and that this theory 

 has, in a very great measure, contriltuted to the advance 

 of embryology. 



Ever since Bnlfotirs time. however, objections have 

 been raised to the germ-layer theory. It has been asserted 

 that it is not applicalile with any absolute conformity to 

 law. For instance, some naturalists have demonstrated 



