18 



kjertelen hos inollusklMrven er en gastruliulannclse (coiifer 

 Braem 7). Vi Imr nemlig hos Åscidii-1 arver ne en distinct 

 niesodermdannclse. der intet har med perihranchialsækkens 

 dannelse at gjore. Denne mesodernidannelse (man betragte 

 ran Beueden and Jnlins prægtige afhandling [3]l optræder 

 meget tidligere i lulvikliiigen og viser stor lighed med me- 

 sodermdannelsen andetsteds, (hvad enten man nu vil antage 

 van Beneden og Jnlins beskrivelse af Ascidielarvens cælora- 

 sække eller ikke). Desuden niaa man dog indrommme, at 

 peribranchialsrekkcn er et saa distinct, specifikt organ, at 

 det vilde være udcn sammenligning i det at se en modifi- 

 ceri't mesodermdannelse. Peribranchiulsækken maa derfor 

 overalt fra et fysiologisk synspunkt opfattes som et eget 

 specifikt organ, og at det hos BotryJlas beklæder tarmtrak- 

 tus ogsaa udenfor pars respiratoria er ikke noget bevis for. 

 at organet indeliolder oprindelig mesodermale anlæg. men 

 alene at forklare af kloakens store udvidelse paa grund af 

 aksefoi-andriniiei-. 



Ethvert holdepunkt for en fysiologisk sammenligning 

 med epicardiet svinder nu end mere ved faktum, at epicar- 

 diet hos Atnaroueinin aldeles ikke beklæder tarmtraktus, 

 men som et direkte cylindrisk rør fortsætter sig ud i post- 

 abdomen. Det er overhovedet et stort sporgsmaal, hvorvidt 

 peribranchialsækken direkte beklæder tarmtraktus selv iios 

 Botrylhts. Af theoretiske grunde skulde jeg være mere end 

 tilboielig til at antage, at der findes fine mesodermelementei- 

 niellem de to organer; men man vil indse, hvor vanskelig 

 det er at afgjore dette, da saavel peribranchialsækken som 

 mesodermelementerne optræder som overordentlig flade plade- 

 epithelier. Jeg tillader mig at anbefale dette til fornyet 

 undersøgelse; frugtbarest vilde utvivlsomt studiet af unge 

 larver og knopper være. (Hvad peribranchialsa-kkens og 

 epicardiets forhold til knopskydningen angaar se Kap. 2). 



Af den foregaaende udvikling mener jeg at kunne 

 udlede følgende resultater: 



1. Peribranchialsækken er hos Ascidia viaiiDniUafa, 

 (Kowalevslcy), Clavelina. (Seeliger, IViUcij. Seelir/er paanyj, 

 Ciona intestinalis (WiUey), Disfajjlia rof-ea, Leptodimtm 

 gclatinosuvi. GlossopJmrum lufeuni. CircinaJium concrescens 

 (CatdJery) et organ, der dannes i emViryonaludviklingen ved 

 to symmetriske ektodermudliugtninger, der siden forener 

 sig til en fælles blære. Jeg foier hertil, at jeg kan be- 

 kra'fte WiJleys resultater, og at jeg i dette arbeide frem- 

 lægger undersøgelser i samme retning over Disfaplia magni- 

 larra. 



2. Hvis pcribranchialsædvkcn hos Bofryllide-lavvevne, 

 som paastaaet af DeUa-YaUe og Pizon samt hos larven af 

 Ascidia viUosa (Pizon) dannes af entodermen, saa staar 

 vi overfor det theoretisk vigtige forhold, at det samme or- 

 gan hos nærstaaende grupper (familier, arter) kan dannes 

 paa v.Tsentlig forskjellig raaade. Af hensyn til sporgsmaa- 

 lets theoretiske betydning turde dette kræve fornyede un- 

 dersøgelser. 



reason a peritoneum auy more than the sliell-ghuid in 

 the MoUusc larva is a gastrula formation (cf. Braem. 7). 

 In Ascidiæ larvæ. we have a distinct mesodernial forma- 

 tion, wiiicii lias nothing to do with the formation of the 

 joeribrancliial cavity. This mesodernial formation (see ran 

 Beneden and Jn]in's capital treatise [3]) appears mucii 

 earlier in the development, and exhibits a great likeness 

 to the mesodermal formation in other groups (whether van 

 Beneden and Jidin's descriptioii of the Ascidian larva's 

 cæloma be accepted or not). It must also be admitted 

 that the peribranchial cavity is sucli a distinct, speeihc 

 organ, that to see in it a moditied mesodermal forma- 

 tion would Ile without [larallel. The periln-ancliial cavity 

 must tlierefore, from a physiological ])oint of view, be re- 

 garded as a i)eculiar, specific organ ; and the fact tiiat in 

 Bofryllus it also lines the alimentary canal outside the 

 pars respiratoria is no proof that tlie organ originally con- 

 tains mesodermal rudiments, but is only to be explained 

 by the great expansion of the cloaca on account of axial 

 variations. 



Every stroiig point for a physiological comparison with 

 the epicardium vanisiies yet more completely when consi- 

 dering the fact tiiat the epicardium in Amaiottcinni does 

 not line the alimentary canal at all. Init is continued as a 

 direct cylindrical tube out into tiie post-abdomen. It is 

 altogether a great question whether tiie iieribranehial cavity 

 forms a dired lining to the intestine, even in Bofryllus. 

 On theoretical grounds. I should be more than inclined to 

 suppose that tiiere are fine mesoderm elements between tlie 

 two organs; l)i;t tiie difficulty of settling this point will he 

 undeistood, as both tlie peribranchial cavity and the meso- 

 derm elements are present as exceedingly tiat. stratified 

 epithelium. I venture to recoramend this question to re- 

 newed investigation. Tlie study of young larvæ and of Inuls 

 would undoubtedly be the most fruitful. (For the relations 

 of the periliranchial cavity and the epicardium to gemma- 

 tion, see Chap. II.) 



From the foregoing evolutioii, I consider tliat the 

 following conclusions may be drawn: 



1. Tiie peribranchial cavity in Ascidia muuitnilJata 

 (Koiualevsl-y), Clavelina (Seeliger, Wilhy and Seeliger againj, 

 Oio)ia intestinalis (Willey), Distaplia rosea, Leptoidiimni 

 gelatinosum, Glossopliornni luteum and Circinaliiini concres- 

 cens (Caidlery) is an organ formed, in tlie emliryonic deve- 

 lopment, by two symmetrical ectodermal evaginations, which 

 subsequently becoine united as a common vesicle. I would 

 here add my confirmation of Willey's results, and that in 

 the present jiaper I give investigations on the same snli- 

 ject in Disfaplia magnilarva. 



' 2. If the peribranchial cavity in Bofryllidw larvæ. 

 as Dilla Talle and Pizon have asserted, and in tlie larva 

 of Ascidia villosa (Pizon), are formed from tlie endoderm, 

 ^ve are confronted by the theoretically important circum- 

 stance, that the same organ can be formed in nearly-allied 

 groups (families, species) in essentially different ways. The 

 theoretical iniportancc of this question demands reneued 

 investisation. 



