hos de stadier, der har gjennemgaaet metamorphoseii. Bagtil 

 viser medullarrøret endnu paa dette stadium (fig. 3) en 

 sammenhæng med entodermen, nogen aaben canahs iieuron- 

 tericus eksisterer ligesaahdt som paa det i fig. 2 afhiklcde 

 stadium. 



Et folgende stadium er fremstillet ved figurerne 5 og 

 (i (Pi. XII): fig. 5 viser en larve seet fra dorsalsideii. fis;. 

 6 eu larve seet fra venstre side. Paa dette stadium finder 

 vi flere vigtige forandringer. Bagtil iiar medullarroret lielt 

 løst sig fra forbindelsen med entodermen, fortil er det 

 vokset sammen med den fortykkede forreste del af gjælle- 

 tarnu'n, der danner ingestionsaabningen. paa overgangsstedet 

 mellem ektoderm og entodermen. Paa dette stadium findes 

 der imidlertid ingen forbindelse mellem gjælletarmen og 

 medullarroret. der er altsaa kun en sammenvoksning af 

 deres vægge. Forovrig viser hjerneblæren sig væsentlig 

 mere differentieret ; udbugtningen af den hoire væg er større 

 og vi finder her allerede ganske store pigmentflekker; for- 

 tykkelsen af venstre væg nærmer sig h.-ilvkugleformen. Lar- 

 verne er ogsaa i andre henseender væsentlig mere udviklede 

 paa dette stadium ; jjeribraneliialsækken er dannet ved sam- . 

 raenvoksning af de to anlæg, og istedetfor de faa gjælle- 

 spalter, der oliserveres paa fig. 3 a liar vi nu 4 rækker 

 færdigdannede gjællespalter; desuden viser ingestions- og 

 egestionsaabningerne sig at være dannede. 



Figurerne 7 og 9 (Pl. XII) viser os nu et stadium, 

 der repræsenterer den voksne larve. Medullarroret er her 

 blevet meget mindre; fortil har der dannet sig en aabning 

 fra medullarroret til gjadletarraen og den forreste del af 

 medullarroret viser sig omdannet i den tragtformige flimmer- 

 grube. Hjerneblærens larvehjerne, som vi saa dannes ved 

 en udbugtning af den hoire væg. iiar nu iielt naaet sin 

 holeste udvikling. Man iagttager de store pigmentflekker, 

 de 3 bekjendte linseceller; paa fig. 'J ser man fremdeles, 

 hvorledes der fra larvehjernens ventrale væ'g udgaar en 

 stor nervemasse, der er forsynet med spredte ganglieceller 

 (først beskrevet af Salensl-//). 



Det er almindelig liekjendt. at denne Ascidiehjernens 

 hoie orgauisation kun varer i larvestadiet og fo]'Svinder 

 under metamorphosen. Fig. 8 (Pl. XII) viser os central- 

 nervesystemet hos det unge fastsiddende dyr. Man gjen- 

 kjender her den sidste rest af medullarroret som det be- 

 kjendte Ascidieorgan. der har faaet navnet Hypophysis (eller 

 rettere Flimmergrube -|- Hypopliysis). Gangliet finder vi 

 beliggende ved siden af hypojjhysen*), medens ,,iarvehjer- 

 nen- helt er forsvunden. I et tidligere arbeide har jeg 

 (31) indga ående beskrevet larvehjernens bygning og forkla- 

 ret, hvorledes dens forsvinden fremkaldes ved en afsnøring 

 af den blæreformig udbugtede hoire væg af det oprindelige 

 medullarror. Efter afsnoringen antager da dette ligesom 

 foran og bag larvehjernen formen af et cylindrisk ror (fig. 

 8, Pl. XII). 



*) Som bekjendt er gangliets beliggenhed i forhold til hypophysen 



forskjelligt hos forskjelUge grupper. 



slight thickening. the earliest indication of the „permanent"' 

 ,?definitive ganglion-, -whicii. as is well known, forms the 

 central organs of tlie nervous system, in those stages Avliich 

 iiave undergone nu-tamorphosis. At this stage (fig 3), the 

 medullary tube still exiiibits posteriorly a connection witli 

 the endoderm, and an opeu neurenteric canal no more 

 e.Kists than in tlie stage figured in fig. 2. 



A subsequent stage is i-epresented in figs. 5 and 6 

 (Pl. XII), fig. 5 shoAving a larva viewed from tiie dorsal 

 side, fig. 6, a larva viewed from tlie left side. At this 

 stage we find several important changes. The medullary 

 tul)e lias quite broken off its connection with the endoderm 

 posterioi-ly, wliile anteriorly it lias coalesced witii the tliick- 

 ened fore part of the l)ranchial gut, which forms tiie oral 

 aperture, at the place of transition from the ectoderm to 

 the endoderm. At tliis stage, however, thei'e is no com- 

 munication between the branchial gut and the medullary 

 tube, only a symphysis of tlieir walls. Besides this the 

 cerel)ral vesicle appears more differentiated ; the protulje- 

 rance of the right wall is larger. aud \ve already find quite 

 large spots of pigment: the tliickening of the left wall ap- 

 proximates a hemisplierical form. The larvæ are more 

 developed at tiiis stage in other respects too; the peribran- 

 ciiial cavity is forraed by the coalescence of the two rudi- 

 ments, and instead of the few branchial stigmata seen in 

 fig. 3 0. we now have 4 rows of fuUy-formed branchial stig- 

 mata ; the oral and atrial apertures moreover appear formed. 



Figs. 7 and !• (Pl. XII) show a stage representing 

 the full-grown larva. The medullary caual has become 

 much smaller: in front an opening lias been formed from 

 it to the branchial gut. and the fore part of the me- 

 dullary canal ai^jjears transformed into the infundihular 

 dorsal tuhercle. The larval cerebral vesicle, wdiich. as we 

 have seen, is formed by a protuberance of the right wall. 

 has now attained to its full development. "We observe the 

 large spots of pigment, tlie 3 well known lenticular cells. 

 In fig. 9 it may also be seen how a large nerve-mass issues 

 from tile ventral wall of the larval braiu. provided witli 

 scattered ganglion cells (first descriljed liy Sali/iish-y). 



It is a generally-known fact that the high orgauisation 

 of the Ascidian brain only lasts througli the larval stage, 

 and disappears during the metamorphosis. Fig. 8 (Pl. XII) 

 shows the central organs of the nervous system in tlie 

 young attached animal. The last reninant of tlie medullary 

 tuIie is liere recognised as the well-know^n Ascidian organ, 

 that has received the name of hypophysis (or rather, the 

 dorsal tuhercle phts the hypophysis). The ganglion is found 

 beside the hypoijhysis*), while the larval lirain has quite 

 disappeared. In a former paper (31), I have carefuUy des- 

 cribed the structure of tlie larval brain, and explained how 

 its disappearance is occasioned by a constriction of the 

 vesicularly evaginated right wall of the original medullary 

 tube. After the separation, this organ assumes, as it were 

 in front of and behind the larval brain, the form of a 

 cylindrical tube (Pl. XII, fig. 8). 



') As is well known, the position of the ganglion iu relation to 

 the hypophysis varies in difiTerent groups. 



