fig. 10). Tarratraktus bugtes ucl som hos de andre grup- 

 per, og bagtil deles den indre blære ved en median sam- 

 menklæbning af den ventrale og dorsale væg i 2 ror, der 

 dog staar i forbindelse med hinanden bagtil. Tversnitserien 

 iig. 2—7, Pl. XI illustrerer dette stadium (Pl. XI, fig. 1). 

 Fig. 2. der er et snit langs linieu u — p' (fig. 1 ) viser i sin 

 midte g^jælletarmen KD, paa dennes sides venstre og hoire 

 peribrancliialsækaulæg, og mellem disse den forreste lukkede 

 spids af dorsalroret. Fig. 3 (langs linien ; — d, fig. 1), 

 viser dorsalroret som virkeligt ror. Fig. 4 og 5 (langs 

 linierne e — i' og i] — H, fig. 1) viser til hoire hvorledes det 

 lioire periln-anchialsækanlæg og til venstre, hvorledes det 

 venstre ]ieril)ranchialsækanlæg og dorsalroret sammen mun- 

 der ud i g;ja41etarmen. Paa disse figurer ser man da lige- 

 ledes at gjælletarnien bagtil gaar over i 2 store rør, knop- 

 pens epicardium. Paa fig. 6 (langs linien i — /., fig. 1) 

 sees epicardiet som 2 ganske adskilte ror. der dog længere 

 bagtil. fig. 7, smelter sammen igjen. 



Man vil ved sammenligning med de tidligere grupper 

 forståa, hvorledes de saaledes beskrevne forhold videre ud- 

 vikler sig til det i fig. 11, pl. X af hildede stadium. Dette 

 er især karakteristiske derved, at de 2 anlreg til peribran- 

 chialsækkeu smelter sammen, at dorsalroret væsentlig for- 

 længes og især smalner af (trækkes ud) bagtil. medens det 

 dog endnu viser en forbindelse med peribranehialsækken og 

 gjælletarmen liagtil. Tarmtraktus er vokset, ligesaa ej)!- 

 cardiet i sit bagre parti. 



Det fig. 12, pl. X af hildede stadium er nu igjen 

 væsentligt ældre. Peribranchialsækkens forbindelse med 

 gjælletarmen er helt afsnoret; ligesaa er dorsalrøret bagtil 

 skilt fra peribranchialsækken. Derimod har det fortil som 

 hos de andre gruj)per forenet sig med gjælletarmen, og 

 man iagttager paa dets forreste dorsale parti en væsentlig 

 fortykkelse til dannelse af gangliet, samtidig med at dets 

 bagre parti er trukket ud til et largt fint ror, der paa 

 tversnit viser sig encellet. Jeg har nu paa talrige snit- 

 serier studeret dorsalroret paa dette stadium og det liar 

 med afgjorende sikkerhed vist sig, at fortykkelsen paa dor- 

 salrørets dorsale side virkelig afsnorer sig til dannelse af 

 gangliet. Figurerne 8 — 11, pl. XI, viser enkelte stadier 

 tra fortykkelsen, fig. 8, til gangliet er helt afsnoret. fig. 11. 

 Specielt er det aldeles bevisende at røret bagtil er encellet, 

 og at dette encellede ror fortil fortsætter sig i en tydelig 

 fortykkelse fig. 8, samt at roret paa a^dre stadier, hvor 

 gangliet er afsnoret, i hele sin udstrækning viser sig en- 

 cellet. Anlægget til hjertet og pericardiet danner sig efter 

 det af van Beneden og ,^ulin givne schema ved afsnoring 

 fra epicardiet. Jeg har ikke anstillet nærmere undersogel- 

 ser herover. 



Koiualevslnjs tidligere undersøgelser stemmer i flere 

 æsentlige punkter overens med denne fremstilling; specielt 



togetlier with the left rudiment of the peribranchial cavity 

 (see Pl. X, fig. 10). The intestine bulges out as in the 

 other groups, and the inner vesicle is divided behind. l)y 

 a median fusion of the ventral and dorsal walls, into 2 

 tubes, which. however, are in comraunication with one an- 

 otlier behind. The series of transverse sections, Pl. XI, 

 figs. 2—7, illustrates this stage (Pl. XI, fig. 1 ). In tlie 

 middle of fig. 2, which is a sectioh along the line « — {i 

 (fig. 1) is the branchial gut KD, at its sides the right and 

 left rudiments of the peribranchial cavity, and between 

 thom, the foremost closed end of the dorsal tube. Fig. ?> 

 (along the line y—d, fig. 1) shows the dorsal tube as an 

 actual tube. Figs. 4 and 5 (along the lines e — i" and j; 

 — 6, fig. 1) show to tlie right, iiow the right rudiment of 

 the peribranchial cavity and to the left. bow the left rudi- 

 ment of the peribranchial cavity aud the dorsal tube to- 

 gether open out into the branchial gut. It will also be 

 seen in these figures that the branchial gut divides Ijehind 

 into 2 large tubes, the epicardium of the bud. In fig. 6 

 (along tile line i — /., fig. 1) the epicardium is visible as 

 2 ([uite sejjarate tubes, wliich. however, again become one 

 behind fig. 7. 



By comparison witli the preceding groups, it will be 

 understood b.ow the conditions thus described develope 

 further into the stage represented on Pl. X, fig. 11. The 

 especially chai-acteristic features about this are that the 2 

 rudiments of tlie peribranchial cavity coalesce, and tliat 

 the dorsal tube is nuich longer, and tapers beliind, while 

 it still communicates tlicre witli tiie periliranchial cavity 

 and the branchial iiut. The alinientary caniil lias grown. 

 as also the epicardium in its liinder part. 



The stage illustrated on Pl. X. fig. 12 is considerably 

 more advanced. The connection of the peribranchial cavity 

 witii the branchial gut is completely broken off, while the 

 dorsal tube is separated behind from the peril)ranchial 

 cavity. In front, on the otiier hand, it has become united, 

 as in the otiier groujis, witli the Ijranchial gut, and on its 

 foremost dorsal part may be seen a considerable thicken- 

 ing for the formation of the ganglion, while its hinder pai"t 

 is di'awn out to a long thin tube, which in transverse sec- 

 tion appears single-celled. I iiave studied the dorsal tube 

 in this stage in numerous sectional series, and it lias ap- 

 peared witli decisive certainty that the tiiickening of the 

 dorsal side of the dorsal tube is really constricted to form 

 the ganglion. Pl. XI. figs. 8 — 11, show cei-tain stages 

 from the thickening (fig. 8), to tiie complete constriction 

 of the ganglion (fig. 11). It is quite convincing that the 

 tube behind is single-celled, and that tiiis single-celled tube 

 is continued in front in a distinct thickening (fig. 8), as 

 also that the tube, in more advanced stages, where the 

 ganglion is constricted appears single-celled throughout its 

 length. The rudiment of the heart and pericardium is 

 formed, aceording to the diagram given by ran Beneden 

 and Jidin by constriction from tiie epicardium. I have 

 made no more detailed investigations on this matter. 



Koivalevslt"if s earlier investigations agree in several 

 essential points with tliis account, especially with regard 



