stnplia rosea, Leptoclinnm f/clafiiiosuiu, GJossophoyiDii lutfnm. 

 Paramdia Giardii. 



Med CutdJerij luaa jeg udtale, at Pizoits fremstilling 

 forekommer mig vanskelig at forståa, og, hvis den er rigtig, 

 da dreier det sig her om en dannelse, som samtlige andre 

 ascidieforskere har ovei-seet, medniindru da PircOii med det 

 ..entodermrør", der vokser frem fra entodermen skulde 

 mene canalis nenrentericus. Denne har imidlertid ingen 

 hjerneblære at vokse hen til. 



Den fremstilling, som er givet i dette arljeide hviler 

 paa en fornyet indgaaende undersogelse af Distaplialarverne 

 og forholdene viste sig her med den størst mulige tydeliglied. 

 For at kunne vise udviklingens forlob mere anskuelig har 

 jeg ogsaa Pl. XII afliihh-t flere stadier end i mit tidligere 

 arbeide. 



Jeg fastholder derfor den tidligeie af WilJeij og mig 

 givne fremstilling og rekapitulerer denne omtrent i de 

 samme ord, som i det forste arbeide. 

 i) Det larvale og det lilivende centralnervesystem frem- 



gaar hos Distaplia af samme anlæg som hypophysen. 



Dette anlæg er den oprindelige, ved udbugtning af 



ektodermen dannede hjerne, — resp. nervebkere (me- 



dullarror). 



2) Larvehjernen staar hos den voksne larve i forbindelse 

 med gjælletarmen (overgangsstedet nudlem den ento- 

 dermale og ektodermale del) gjennem liypofysens for- 

 reste del, fiimmergruben. 



3) Hypofysens forreste del dannes direkte af medullar- 

 roret (foran hjerneblæren), den bagre del at hjerne- 

 blærens venstre vreg. Hypofysen er derfor hos det ud- 

 voksede dyr (efter metamorfosen) den eneste direkte 

 rest af det oprindelige medullarror. 



4) Det blivende ganglion danner sig som en fortykkelse 

 af hjerneblærens venstre epithelvæg. 



Vi nævnte ovenfor, at idet ascidielarven efterhaanden 

 ved en indsnoring udviklede et forreste bredere og et bagre 

 smalere parti (halen) optraadte ogsaa de første anlæg til 

 chorda og mesoderm. Dette foregaar paa den nmade, at 

 den oprindelig blæreformige entoderm i det smale haleparti 

 mister sit lumen og derved gaar over til en kompakt dob- 

 beltlaget cellemasse. Denne sondrer sig nu i medianlinien, 

 dorsalt i chorda, ventralt i den ..rudimentære entoderm- 

 streng", medens nn.'sodernien afsondres til begge sider. (Se 

 tekstfigurerne 2—5). Paa det i fig. 5 afbildede stadium 

 har saaledes halen allerede i det væsentlige sin karakteri- 

 stiske bygning, dorsalt en fortsættelso af medullarroret, 

 under dette chorda, ventralt entodermstrengen.*) 



(9), who examined CircinaJiitin concrescens, Didaplia rosea, 

 Leptoclinuiii gélaiinomm. GIo.-'$np)ioy>nu Juteum and Paras- 

 cidia Giardii. 



I nnist agree witli CauUery in saying tliat Pi.t(iii's 

 description seems difticult to understand, and if it is cor- 

 rect, there is heri' (juestion of a formation, which all other 

 investigators of Ascidians have overlooked, uidess by tiu' 

 „endodermal tul)e-' growirig out of the endoderm, Pizoit 

 means the neureuteric canal. The latter, however, has no 

 cerebral vesicle to grow up to. 



The description given in this paper is based u])on a 

 renewed thorougji exaniination of DistapJia larvæ, and the 

 circumstances api)earod here with tbe greatest possible dis- 

 tinctness. In order to show the course of development 

 more plainly, I iiave given, on Pl. XII, a greater numl)er 

 of stages than in my jirevious paper. 



I therefore hohl to the aecount previously given liv 

 Wille/j aud myself. recapitulating it almost in the \voi'ds 

 of the former paper. 



(1) The larval and the pernjauent uerve-centres originate 

 in Distaplia in the same rudiment as the hypophysis. 

 This rudiment is the original 1n-ain formed by an inva- 

 gination of the ectoderm — - nerve vesicle (medullary 

 tube). 



(2) In the full-grown larva, the larval brain is connected 

 with the branchial gut (the place of transition from 

 the endodermal to the ectodermal part) through the 

 front part of the hypophysis, the dorsal tuberele. 



(3) The anterior part of the jiypophysis is formed directly 

 from the medullary tube (in front of the cerebral ve- 

 sicle), tlie posterior part from the left wall of the 

 cerebi-al vesicle. The hypophysis in the full-grown animal 

 (after metamorphosis), is therefore the only direct rem- 

 nant of the original medullary tube. 



(i) The iiermanent ganglion is formed as a thickening of 

 the left epithelial wall of the cerebral vesicle. 



AVe mentioned above, that as the Ascidian larva, by 

 a constriction, gradually developed an anterior broad part, 

 and a posterior narrower part (the tail), the earliest rudi- 

 ments of the notochord and the mesoderm also made their 

 appearance. This takes place in the following manner. 

 The original vesicular endoderm in the narrow tail portion, 

 loses its lumen, and thus turus info a eompact, bi-huninar, 

 cellular nuiss. Tiiis now divides in the median line, dor- 

 sally into the notochord, and ventrally into tbe ..rudimen- 

 tary endodermal cord", while the mesoderm is separated 

 at both sides. (Sec text-figures 2 — 5). In the stage re- 

 presented in fig. 5, the tail has already aequired its essen- 

 tially characteristic structnrc, dorsally a continuation of the 

 medullary canal, beneath this the notochord. and ventrally 

 the endodermal cord. *) 



*) Ell iiæi-mere beskrivelse af cliordas udvikling og mesodermens ') A fuUor description of the development of tlic notochord, and 



omdauiielso til mnskulatur ligger udenfor vor opgave. the conversion of the mesoderm into muscidature, lies beyond 



tlie scope of our present subject. 



Den norske Nonlliavse.-spedition. Johan Hjort: Kinibladstuilier paa grumllijf af Aseidiernes udvikling. - 



