62 



den & Jitlin, Seeliger og Willey og disse forskere har i 

 høieste grad vist, hvorledes Ascidielarven er en ganske 

 anderledes lioit organiseret form end det voksne dyr. 



Det ligger nu helt lulenfor min opgave at ville levere 

 noget bidrag til Ascidiernes fylogeni, raine studier har ikke 

 været rettet mod dette maal og jeg lienviser i den anled- 

 ning til de ovennævnte forfattere. Derimod vil jeg med 

 faa ord omtale den i Kap. 4 anstillede sammenligning mel- 

 lem larve- og knopudvikling i almindeligliod. og soge at !{ 

 lielyse de der vundne resultaters forhold til den l)iogene- 

 tiske grundlov. 



Vi saa i Kapitel 5, at knopskydningen væsentlig af- 

 veg fra larveudviklingen deri. at den 



1) var en forkoi-tning af larveudviklingen. 



2) at organdannelsen foregik efter en væsentlig anden 

 plan end embryonaludviklingen. 



Der fandtes i knopudviklingen intet gastrulastadinm : 

 hverken sandseblæixni, chorda, hale etc. kom til anlæg. 

 Seeliger siger (57. [tag. 45) meget træffende: ,,Gerade die 

 Stadien fehlen, auf welche . die Wirlieltliierverwandtschaft 

 begriindet wurde.-' 



Vi staar saaledes her overfor det eiendommelige for- 

 hold, at en grupjje kau have 2 udviklingsmodi, af hvilke 

 (len ene efter forskernes mening rekapitulerer, den anden 

 ikke rekapitulerer alle de stadier, der miuder om hvirvel- 

 dyrencs bygning. Seeliger, der synes at have havt sin op- 

 merksomhed specielt henvendt paa dette punkt, søger fylo- 

 genetisk at forklare dette saaledes, at Ascidierne først 

 begyndte at kuopskyde, efterat de hvirveldyrlignende stadier 

 (stadier i fylogenetisk betydning) var passeret, altsaa som 

 fastsiddcnde dyr. 



Efter analogi med andre fylogenetiske theorier kan 

 jeg tænke mig, at man vil søge at forklare det faktum, at 

 knopudviklingen ikke viser de hvirveldyrlignende stadier, 

 paa eu af 2 maader. 



Enten vil man mene. at larvernes udvikling virkelig 

 er rekapitulation, men at knopperne ikke ..Ijehover" at re- 

 kapitulere dem, da de fra liegyndelsen af er fastsiddende 

 og ikke har noget larvestadium, i hvilket de kunde bi-uge 

 sin larvehjerne og sin hale. Sammenlign Laiigf interes- 

 sante : Ueber den Einfluss der festsitzenden Lebensweise 

 auf die Thiere (41 1)). 



Eller ogsaa vil man sige, at larven ikke er eu reka- 

 pitulation af den fylogenetiske udvikling, men alene repræ- 

 senterer en tilpasning til det fritsvomnu-nde stadium, der 

 er af tordel for artens udbredelse, men som ikke er af 

 l)etydning for knopperne, der danner kolonierne. 



Begge disse theorier slutter derfor, at hvis en reka- 

 pitulation overhovedet finder sted hos Ascidierne, sau Jatn 

 den dog helt bortfalde, naar den ikke er af direkte hetydning 

 for indiridernes eksistents paa det ene eller andet stadium. 



I Kapitel 4 saa vi, at (70sfr«fø-stadiet forholdt sig 

 l)aa lignende maade. Det lykkedes os ikke i knopudvik- 



fully studied the development of Ascidians. Among them 

 I would specially uame v. Beneden and Jtdin. Seeliger aud 

 Willey, and these anatomists have shown most distinctly 

 tiiat the Ascidian larva is a far more higlily organised 

 form thau the adult animal. 



It is quite outside the scope of my subject to make 

 any contriljution to the phylogeny of Ascidiæ ; my studies 

 have not been directed towards that object, and on tiiat 

 head, I would refer to the above-named authors. On the 

 other hand I will briefly discuss the coniparison put for- 

 ward in Chapter IV, between larval and bud development 

 in general, and try to throw sorae light on the relation of 

 the results there gained, to the biogenetic fundamental law. 



AVe saw in Chapter IV that the cluef differences be- 

 tween bud and larval development were: 



(1) That buddiug was au ablireviation of tlie larval 

 development ; 



(2) That tiie formation of tiie organs proceeded on 

 an essentialh' different plan in the two modes of development. 



In budding there was no gastrula stage, no rudiments 

 of sensory vesicles, notocliord, tail, etc. Seeliger very aptly 

 says (57, p. 45): ..Geråde die Stadien fehlen, auf welche 

 die Wirbelthier-verwandtschaft l)egrundet wurde." 



AVe thus stand face to face witli tlie peculiai' fact of 

 a group having two methods of develo])ment, one of which, 

 accordiug to the opinion of anatomists, recapitulates, and 

 the other does not recapitulate all the stages tiiat recall 

 the structure of tlie vertebrate animal. Seeliger, wlio seems 

 to have had his attention especially turnod to this point, 

 seeks to explain it phylogenetically, by saying that the 

 Ascidiæ did not begin to produce buds until after the 

 vertebrate-like stages (stages in the phylogenetic sense) were 

 jjassed, in other words not until they became stationary 

 animals. 



After analogy with other phylogenetic theories, I can 

 fancy that attempts will be made to explain in one of two 

 ways, the fact that Ijudding does not show the vertebrate- 

 like stages. 



It will either be held tiiat the development of the 

 larva is really recapitulation, but that the buds do not 

 ..need" to recapitulate, as they are attached from the very 

 first, and liave no larva stage in which they can use their 

 larval brain and their tail (cf. Lang's interesting ,,Uel3er 

 den Einfluss der festsitzenden Lebensweise auf die Thiere- 

 [41b]); or it will be said that the larva is not a recapi- 

 tulation uf the phylogenetic development, but only repre- 

 seiits an adaptation to the free-swimming stage, which is 

 advantageous to the spread of the sjjecies. but is of no 

 importance to the buds whicli form the colonies. 



Both these theories couclude therefore, that if, on 

 the whole, a recapitulation takes place in Ascidians, it may 

 entirely be oniitted when of no dired impoiiance to tlie 

 existence of tlie animal in one stage or tite otJier. 



In Chapter JX, we saw that tlie gastrula stage was 

 very siniilar. We did not succeed in tinding, i'.i the bud- 



