22 



sæk kunde freui')ringe den saiUiiK- dauiielso, og disse tvivl 

 bestyrkedes end niere derved, at de embryologiske undcr- 

 sogelser med overveiende samstemmiglied beskrev pcribran- 

 cliialsækken som en ektodermal dannelse. Problemet til- 

 spidsedc sig derefter mere og mere i det sporgsmaal : 

 „Kan ektodermen og entodermen saaledes give anledning 

 til anlægget af den samme formdannelse?" Ved nærmere 

 betragtning fandt man det fremdeles uantageligt, at Botryl- 

 hisJiuoppen kunne udvikles ud fra to ektodermale blærer, 

 altsaa uden „entodermalt materiale". 



Pizon har g,jort et særdeles interessant forsog paa at 

 tinde overensstemmelser mellem alle knopskydninger med 

 hensyn til det ..kimbladmateriale", hvoraf de dannes. Hans 

 undersogelser forte liam. som jeg nævnte i Kap. I, til den 

 opfatning, at periljranciiialsækken hos BotrijJlideriies larver 

 ikke dannes som to ektodernialudbugtninger, men som to 

 udbiigtninger af entodermen, der siden forenede sig til en 

 fælles peribranchialsæk. Fra denne dannede der sig nu 

 senere to udinigtninger, der omklæder tarmen, og disse 

 kaldte han e^jicardiahulhitgtninffer. Da nu knopperne lios 

 Botrylliderne i regelen opstaar i moderdyrots bagre krops- 

 pai'ti. saa mente han, at knopskydningen ogsaa hos Botryl- 

 liderne udgik fra entodermen og det endog fra epicardial- 

 dannelserne. Da nu fremdeles epicardialdannelserne dels 

 direkte (Amaronc/iiiii, Clavelina) dels indirekte {Batryllm) 

 var entodermdannelser saa fandtes der ingen forskjel med 

 hensyn til knopanlægget hos de forskjellige sammensatte 

 Ascidier. Overalt dannedes knopanlægget af en indre en- 

 todernial og en ydre ektodermal blære, ligesom gastrula- 

 stadiet i enibryonaludviklingen liestod af to blærer, den 

 ydre ektodermen, den indre entodermen. 



Det er indlysende. at denne o[)fatning staar og fakler 

 med de udviklingsiiistoriske undersogelser over emhryonal- 

 udvildingen. Vi saa da i kap. I, at den embryologiske 

 litteratur med afgjorende samstemmighcd forte til det re- 

 sultat, at peribranchialsækken iallefald hos de fleste Ascidie- 

 familier var en i-en ektodermaldannelse (jeg minder om 

 Koivalev^l-ys, Seeligers, WiJley^. Canllerys og egne under- 

 sogelser). Jeg drog heraf den slutning, at man enten imod- 

 sætning til Pizon maatte antage at peribranchialsækken 

 ogsaa hos Botrylliderne var en ektodermaldannelse, eller at 

 vi stod over for det — theoretisk vigtige — forhold, at 

 l)eribrancliialsa'kken hos den ene larve dannede sig fra ek- 

 todermen, hos den anden fra entodermen. Jeg gjorde frem- 

 deles opmrerksom paa, at man under enhver omstæ>ndighed 

 ligegyldig hvorledes peribrancliialsækkcn dannedes, dog ikke 

 kunde kalde endel af dette organ for epicardiwn, idet dets 

 specielle funktion og bygning var altfor forskjellig fra de 

 typiske epicardi.aldannelser hos f. eks. Amarouciuiii. 



Det er selvfolgelig tvivlsomt, hvorvidt man har beret- 

 tigelse til at l)enegte rtgtigheden af en videnskabelig under- 

 søgelse udfort af en dygtig forsker som Pizon. hvor man 

 selv ikke har egne erfaringer fi-a samme objekt, og onsker 

 jeg mindst af alle af ensidige theoretiske synspunkter at 

 fores til et falsk resultat. Jeg tillader mig imidlertid at 



canal and the peribranehial cavity could produce the same 

 formation, and this doubt was strengthened vet more Ijy 

 the great unanimity with which embryological investigations 

 described the peribranehial eavity as an ectodermal forma- 

 tion. The problem then resolved itsclf more and more 

 into tile ipiestion: Can the eetoderm and tlie endoderm 

 give rise to the same form formation? On further consi- 

 deration it was still deemed inadmissible, tliat the Botryl- 

 lus hud could be developed from two ectodermal vesicles, 

 or, in other words, without ,,endodernial material-'. 



Pizon lias made an especially interesting atterapt to 

 dnd iiarmoiiy between all kinds of budding, as regards the 

 gerni-layer material of wiiicli they are formed. His inves- 

 tigations led him. as I stated in Chapter I, to the opinion 

 tliat the peribranehial cavity in Botryllidæ larv.æ is not 

 formed in the shape of two ectodermal invagiuations, l.>i.it 

 in the siiape of two evagiuations of the endoderm. which 

 afterwards unite to form a common peribranehial cavity. 

 From this are subsequently forined two evaginations, which 

 encase the intestine, and tliose he called e2)icardicd evayi- 

 nation.i. Now, as the buds in Botryllidæ originate, as a 

 rule, in the posterior part of the parent-aniniaFs liody, 

 Pizon considered tliat ])udding. in Botryllidæ too, originated 

 in the endoderm, and even in epicardiac formations. As 

 epicardiac formations. some direct {Amaroticiiim, Clavelina) 

 sonie indirect (Botryllns). were still endodermal formations, 

 there was no difFerence, as regards the origin of the bud, 

 in the various Comjiound Ascidians. The incipient bud 

 was ahvays formed from an inner endodermal, and an outer 

 ectodermal vesicle, just as the gastrula stage in the eni- 

 bryonic development consisted of two vesicles, the outer, 

 the eetoderm, the inner. the endoderm. 



It is obvious tliat this theory is dependent upon the 

 developmental-liistorieal investigations in emhryonic develop- 

 ment. We saw in Chapter I, that embryological iiterature. 

 with decisive unanimity, pointed to the conclusion tliat the 

 peribranehial cavity, at any rate in most Ascidian families, 

 was purely an ectodermal formation. (I would call to 

 mind Kowalevskys, Seeliyers. Willeys, Caullery'ti and my 

 own investigations). From this I drew the conclusion that 

 either, in opposition to Pizon, we must presume that the 

 peribranehial cavity in Botryllidæ too, is an ectodermal 

 formation, or that we were facing the theoretically impor- 

 tant circumstance, that in the one larva, tiu' i)erihrancliial 

 cavity was formed from the eetoderm. in the other froin 

 the endoderm. I still called attention to the fact, that 

 under any circumstance, no matter liow the jieribranchial 

 cavity was formed, a ])art of that organ could not be 

 called the cpicardium, as its special function and structure 

 diftered too niueh from the typieal epicardiac formations 

 in, e. g., Amarouciuui. 



It is of course doubtful how far one is justified in 

 deiiying the correctness of a scientific investigation made 

 by an able naturalist like Pizon. wlien one has no perso- 

 nal experience of the same subject ; least of all would I 

 wish to be led, by one-sided theoretical points of view, to 

 a wrong conclusion. I venture, how'ever, to propose the 



