45 



den fores ind i knoppen fra nioderdyrct som vandrende 

 Ijindevævsceller (Se fig. lo). 



Dette forhold, at enkelte organer, der iorekonuner 

 hos larven, ikke anlægges hos knoppen, har fort til. at man 

 har kaldt knopskydningen en forkoiiet larveudvikliiKj. Saa- 

 ledes siger ran BcnPilen og Jidhi (3. pag. 323): ..Mais il 

 est incontestable. (jue Fétude de la larve seule peut fournir 

 des données certaines pour Tinterpretation des processns 

 généti(iues priraitifs: levolution dn bourgeon est pins di- 

 recte". Dette udtrykkes ogsaa ahnindelig saaledes: knop- 

 udviklingen er ikke i den grad som larveudviklingen en 

 rekapitnlation af den fylogenetiske udvikling Vi kommer 

 siden tilbage til dette pnnkt 



Knopndviklingen lader sig imidlei-tid ikke helt ud 

 forklare som en ..kortere embryonaludvikling". Ogsaa den 

 maade, hvorpaa organerne dannes, viste sig nemlig efter 

 de specielle kapitler at van'e va^sentlig forskjellig hos knop 

 og larve. 



Den største interesse ved en saadan sammenligning 

 er knyttet til nervesystemet og peribranchialsiekkens dan- 

 nelse i de 2 ndviklingsmodi. 



Vi saa, at centralnervesystemet og hypophysis hos 

 larverne fremgik af mednllarroret, en dannelse hvis anlægs- 

 niaade er almindelig kjendt paa grnnd af de store ligheder 

 det viser med centralnervesystemets dannelse hos hvirvel- 

 dyrene (se forøvrig pl. XII, fig. 1 og 2 samt de i kap. 1 

 optagne tekstfigurer). Mednllarrøret er altsaa en ren ek- 

 todermal dannelse, det fremgaar af gastrnlastadiets ,.ydre 

 blære". Vi saa deniæst ved beskrivelsen af de paa pl. XII 

 afbildede figurer, at mednllarroret under den videre udvik- 

 ling sondrede sig i den larvale hjerne med dens sandse- 

 organer (tilhoire). det blivende ganglion (dannet ved fortyk- 

 ning af medullarrorets venstre væg) og hypophysen, der 

 repræsenterer mednllarroret i meget rudimentær form, efterat 

 bjerneblæren og ganglion har løst sig fra det. Hvorledes 

 denne sidste udviklingsproces foregaar fremgaar instruktivt 

 af pl. XII. Man ser fig. 3, at mednllarroret fortil ender 

 blindt, fig. 5 og G at det er vokset sammen med gjælle- 

 tarmen (dennes formtarmparti) og fig. 7 at roret kommuni- 

 cerer med gjælletarmen. Figurerne IJ. 7 og 8 demonstrerer 

 endvidere gangliets og hjerneldærens successive udvikling. 

 Ved at sammenholde fig. 7 med fig. 8 forstaar man. hvor- 

 ledes det blivende centralnervesystem under metamorphosen 

 dannes af det larvale centralnervesystem. idet nemlig hele 

 j.larvehjernen" afsnores og resorberes. 



Væsentlig anderledes var nu centralnervesystemets 

 dannelse hos knopperne. Her dannes det ikke af anlæggets 

 ydre blære men af den indre. Istedetfor at være en ind- 

 bugtning, som hos larverne, er det en udbugtning af den 

 indre blære. Sammenligningen mellem de forskjellige asci- 

 diegrnpper viste, at anlægget til centralnervesystemet, som 

 vi her med et indifferent navn kaldte ..dorsalroret" efter 



the endoderm; but it is brought into the bud from the parent 

 animal as wandering connective-tissue cells (see fig. 13). 



The circumstance tiiat of certain organs øccurring in 

 the larva, there are no rudiments in tlie bud. has led to 

 Imdding being called a sliortened larvnl deveJojiDient. Thus 

 ran Beneden aud JuJin (3, p. 323) say: ,,Mais il est in- 

 contestable que Tétude de la larve seule peut foui-nir des 

 données certaines pour Tinterprétation des processus géné- 

 tiques primitifs: révolution du bourgeon est plus directe". 

 This may generally be expressed: bud development is not, 

 to such an extent as larval development, a recapitulation 

 of the phylogenetic development. AVe shall return to this 

 point later on. 



Bnd development does not. however. admit of lieing 

 interi)reted entirely as a shorter embryonic develoinnent. 

 The way too. in which the oi'gans are formed appeared, 

 as shown in tlie special chapters, to be essentially different 

 in l)ud and larva. 



The greatest interest in such a comparison is atta- 

 ched to the formation of the nervons system and the peri- 

 branchial cavity in the two modes of develo])meut. 



We have seen tliat the central organs of the nervous 

 system, and the hypophysis in the larva proceed from the 

 medullary tube, a fcn-mation of which the manner of origi- 

 nating is generally known on account of its great resem- 

 blance in some points to the formation of the central organs 

 of the nervous system in verteljrate animals (see also Pl. 

 XII. figs. 1 and 2; and the text-figures in Chapter I). 

 The medullary tube is thus a purely ectoderraal formation, 

 proceeding from the outer vesicle of the gastrula stage. 

 We then saw liv the description of the figures on Pl. XII. 

 tliat the medullary- tube, during its further development, 

 was divided into the larval brain witii its organs of sense 

 (on the right), the permanent ganglion (formed by the 

 thickening of the leit wall of the medullary tube) and hy- 

 pophysis, which represents the medullary tube in a very 

 rudimentary form, after the cerebral vesicle aiul the 

 ganglion have detached themselves from it. How this last 

 developmental pi-øcess takes place will be at once seen 

 from Pl. XII. Fig. 3 shows that the medullary tube ends 

 cæcally in front; figs. 5 and 6, that it has coalesced witli 

 the branchial gut (the fore-gut part); and fig. 7 that the 

 tube commuuicates with the branchial gut. Figs. 6, 7 and 

 8 further demonstrate the successive development of the 

 ganglion and the cerebral vesicle. By comparing fig. 7 

 with fig. 8, it \vill be understood bow the permanent cen- 

 tral organs of the nervous system during metamorphosis 

 are formed from the central organs of tlie nervous system 

 in the larva, the -whole of the larval l)rain Vteing constricted 

 and absorbed. 



The formation of the central organs of the neiTOUS 

 svstem in the bud is essentially difl^erent. They are not 

 there formed from the outer, lint from the inner vesicle of 

 the rudiment Instead of being an invagination, as in the 

 larva, they are an evagination of the inner vesicle. Com- 

 parison between tlie various Ascidian groups showed that 

 the rudiment of the central organs of the nervous system, 



