2o 



djjkasti' det si)orgsiiiaal til foi-iiyot oa: gruiidigeiT' overveielse: 

 ,.Hva(l er mest uoverensstemiiieiule med den alraiiulelige 

 eiubryologiske erfaring og med den saakaldte i<iml)ladirere. 

 enten den antagelse, at i)eribranclii<dsæl;l;en iios forskjellige 

 familier anlægges fra forskjellige kimblade i embryonalud- 

 viklingen, eller den opfatning, at begge kimblade endog 

 sent i udviklingens forlol) formaar i lige grad ved kno[)skyd- 

 Iling at danne nye individer?-' 



Som det nærmere vil blive gjenstand for l)etragtning 

 i dette arbeides generelle del, er kimbladlæren fremforalt 

 en generalisering af den almindelige embryologiske erfaring, 

 at bestemte organer, som f. eks. centralnervesystemet, i 

 embryonaludviklingen kommer til anlæg og udvikling paa 

 den samme maade og fra det samme primære kimblad 

 overalt i dyreriget. Fra denne regel forekommer der vist- 

 niik mindre undtagelser. der viser, at udviklingen heri be- 

 siddi'r en vis modificerbarhed, men en saa stor afvigelse fra 

 regelen som den, at et oi-gaii hos en familie dannes fra 

 ektodermen, og hos en anden familie af samme grupjje fra 

 entodermen, er dog ganske ukjendt. Skulde dette dog vise 

 sig at være tilfældet, hvad man maaske ikke tor benægte 

 paa forhaand, saa vil det være et nyt og vigtigt bidrag til 

 forstaaelsen af, inden Jivilhe grændser embryonaludviklingen 

 ei' bunden til en bestemt lovmæssighed. Under enhver oiii- 

 stændighed maa imidlertid ikke et saadant forhold anføres 

 for cd vise at der i AsridiendvikJliifjeii ilihe forekoiiiiiier 

 afvigélscr fra Icindiladlæren. 



Betragter vi nu knopankegget og knopskydningen, saa 

 forekommer det mig under enhver omstændighed, selv om 

 peribranchialsa-kkeii hos Botrylluslarven er et entodermalt 

 organ, at man ikke hos nogen grujipe kan kalde knoiian- 

 læggets indre blære et entodermalt anlæg. Dels viser nem- 

 lig den videre knopudvikling (se Kap. 3), at denne indre 

 blære danner de mest forskjellige organer, og dels er dog 

 knopanlægget, saasnart det eksisterer som saadant, éi nyt 

 individ. Ligesaalidt som man derfor hos hvirveldyrene 

 kakler centralnervesystemet et mesodermalt organ, skjont 

 det dannes af ægget, der engang var en mesoderraal celle, 

 saa maa man ogsaa for knoijpernes vedkommende undlade 

 at „projicere-' dem tilbage paa moderdyrets organer. Xnop- 

 pens forskjellige dele faar betragtes i forhold til den hele 

 knops egen bygning. Det viser sig da. som vi siden 

 skal se. at knoppen aldeles ikke har kimblade; hvis den 

 har det, saa er de (de to blærer) andre slags kimblade end 

 hos larven, og knopudviklingen har derfor intet med kim- 

 bladlæren at gjore. Kimbladlæren er dannet for larveud- 

 viklingen og kerer, hvorledes larven opbygges af det be- 

 frugtede æg. Kun paa et punkt berorer kimbladlæren læren 

 om knopperne. Materialet til knojjperne kommer ira lar- 

 vens kimblade, ikke i form af kimblade men i form af 

 enkle epitlielmembraner. Dette forhold lærer os altsaa ikke 

 noget om den egentlige embryonaludvikling og dens forlob, 

 men det viser os kun, at der i kimbladene, ektodermen 

 som entodermen under udviklingen tindes materiale, der 

 naar det kommer vnder andre og nye forhold, naar det 



following question An- renewed and more thorough consi- 

 deration: Which is the more at variance witli gener.al eiu- 

 liiTological experience. and with the so-called germ-layer 

 theory, the supposition th.nt the i)erihranchial cavify. in 

 different families, originates in different germ-layers in the 

 enibryonie develoi)ment ; or the theory tiiat both germ- 

 layers, even if lati' in the course of development, are equally 

 capaljle of ibrming new individuals by budding? 



The germ-layei' theory, as will be sbown in tiie fai-- 

 tlier eonsideration of tlio subject in the General Section of 

 this pai)er, is above all a generalisation of the ordinary 

 embryological e.xperience tliat certain organs, as, for in- 

 stance, the central organs of the nervous system, in the 

 embryonic development, originate and develo])e in the same 

 manner, and from the same primary geiun-layer. throughout 

 the animal kingdom. It is true. tliere are soine small ex- 

 ceptions to this rule, wliich show that the development 

 herein possesses a certain cai)ability of modificatiim, l)ut 

 so great a departure ln)ni the rule, as that an organ in 

 one family is formed from the ecloderm. and in another 

 family of the same group. from the endoderm. is quite un- 

 heard-of. Should this, however, prove to be the case, and 

 perhaps one cannot venture to deny it at the outset, it 

 will be a new and important contribution to the compre- 

 hension of the ciuestion as to ivitliin irhat limits eniliryonic 

 development is bound to a fixed conformity to law. In 

 any case, however, such a circumstance must not lie brought 

 forward to show that in Aseidian development iliere are no 

 deriations from the germ-layer theory. 



If \ve consider tlie commenceiiient and jjrogress of 

 the bud, it seems to me that under any circumstance, even 

 if the pcribranchial cavity in the Boiryllus larva is an en- 

 dodermal organ, in no group can we call the inncr vesicle 

 of the inciiiient bud an endodermal i-udiaient. The further 

 develojiment of the bud partly shows (see cliap. III) that 

 this inner vesicle forms the most various organs, and yet, 

 to a certain extent, tiie rudiiuentary bud. as soon as it 

 comes into existence as such, is « neiv animal. AVe are 

 no more entitled to ,.project" the origin of the buds back 

 to the organs of the parent animal, tlian we are. in the 

 case of vertebrate animals, to call the cerebro-spinal tube 

 a mesodermal organ, although it is formed from the ovum, 

 which was oiice a mcsodenn cell. The various parts of 

 the bud must be considered in relation to the parti- 

 cular structure of the wliole bud. It then aiipears, as we 

 shall subsequently see. that the bud has no germ-layers at 

 all; if it has, then they (the two vesicles) are a difiereut 

 kind of germ-layer to that of the larva, and the develop- 

 ment of the bud has therefore nothing to do with the 

 germ-layer theory. The germ-layer theory is formed for 

 larval develoinnent. and teaches how the larva is formed 

 from the impregnated ovum. Only on one point has the 

 germ-layer theory any connection with that of buds. The 

 material for the buds comes from tlie germ-layers of the 

 larva, not in the form of germ-layers, but in that of simple 

 epithelial membrancs. This circumstance, therefore, does 

 not teach us anything abnut the actual embryonic develo])- 



