14 



rolle i knoppens udviklingshistorie, idet de foruden 

 at danne anlægget til iiefvcsystemet (se nedenfor), 

 ogsaa danner Jcloakken og er delagtige i dannelsen 

 af ffjællespalterne. 



h. Nervesijstemet, der anlægges fra peribranchialcavi- 

 teterne ved at disse, som paa tidligere stadier var 

 fuldstæudig adskilte, forbindes ved et ror under 

 entodermroret paa knoppens proximale side. medens 

 samtidig paa hver side en tapformet cellestreng 

 vokser opad fra peribranchialcaviteten for senere at 

 smelte sammen over entodermroret. Her udvikler 

 de sig til eu stor cellehob, som snart faar en spalte 

 i sit indre, ligesom de to cellestrenge ogsaa lidt 

 efter lidt gaar over til ror, og man har nu et 

 ringformet dorsalror, som omslutter entodermrøret 

 paa dettes overgang til den indenfor liggende knop. 

 Forbindelsen med ijeribrancliialcaviteterne afsnores, 

 de to sidegrene og deres forbindelse under tarmen 

 mister sit himen og gaar over til nerver, medens 

 ganglion, hypofysis og flimmergrube udvikles af 

 nerverørets øverste del. 



c. TarDilriualr» anlægges tidhg som to udbugtninger 

 fra entodermrørets underside. Efterhvert som knop- 

 pen vokser, og især eftersom peribranchialcaviteterne 

 udbreder sig i knoppens proximale del for at danne 

 kloakken, trænges disse udbugtninger frem i den 

 distale ende af knoiipen. Ved indbugtninger af 

 entodermrovets distale del, over og paa siderne af 

 tarmanlægget, afsnøres dette delvis fra gjællesæk- 

 ken. idet der kun paa høire side vedligeholdes en 

 aaben communication [æsopliafiui^)^ medens udbugt- 

 ningeu paa venstre side bliver til den egentlige 

 tarm. 



Som det fremgaar af ovenstaaende. sker udviklingen 

 af knopperne hos Pj/rofoma efter væsentlig de samme love 

 som ogsaa er gjældende ved SynascidierneK knopskydning. 

 Den ydre blære tager ingen del i knoppens udvikling; men 

 de vigtigste organer dannes fra den indre blære. 



At peribranchialcaviteter og nervesystem ikke. som 

 hos Synascidierne dannes ved en simpel udbugtning af den 

 indre blæres væg. men ved at kun en liden del af væggen 

 bugter sig ud. tinder sin forklaring i cellelagenes store 

 tykkelse. 



Dette, at cellevæggeue beholder sin tykkelse frem 

 gjennem knoppernes udvikling. istedet for som hos Disiap- 

 lia at trækkes ud til lavt cylinder- eller pladeepithel, gjor 

 undersogelsen af Pyrosoina meget vanskelig, da væggeue af 

 de forskjellige organer derved, specielt i knoppens dorsale 

 l^arti, kommer til at ligge tæt op til hinanden. Uheldig 



tracted. Tlie peribranchial cavities play an impor- 

 tant i>art in the developmental history of the bud. 

 inasmuch as besides forming the origin of the nervoux 

 system (see below). they also form the cloarn. and 

 share in the formation of the hmncliial stiymata. 



h. The nervous system, which is formed from the peri- 

 branchial cavities in the following manner. These 

 cavities. which in previous stages were quite sepa- 

 rate, are connected by a tulie beneath the endo- 

 dernial tube on the dorsal side of the bud; while 

 at the same time a process-like cellular cord grows 

 upwards on each side from the peribranchial cavity, 

 the two cords merging into one later on above the 

 endodermal tube. There they develope into a large 

 cellular mass, inside which a cleft soon appears, 

 while the two cellular cords also change gradually 

 into tubes; and there is now an annular dorsal 

 tube, surrounding the endodermal tube on its pas- 

 sage to the bud lying within. The connection with 

 the peribranchial cavities is broken off, the two 

 lateral rami and their connection under the intes- 

 tine lose their lumen, and become nerves, while the 

 ganglion, the hypophysis and the dorsal tubercle 

 are dcvehipcd from the upjier part of the nerve 

 tube. 



r. The /utest/nal niiicd eommences to form early in 

 the shape of two evaginations from the infcrior 

 side of the endodermal tube. As the bud grows, 

 and especially as the peribranchial cavities expand 

 in the proximal part of the bud to form tbe cloaca, 

 these evaginations ai-e forced forwards into the 

 distal end of the bud. By the pressing in of tlic 

 distal portion of the endodermal tube above. and 

 at the sides of the rudimeutary intestine. the latter 

 is partially separated from the branchial sac, an 

 open communication (the æsophagus) being main- 

 tained only on the right side, while the evaginatiou 

 on the left side becomes the intestine prni)er. 



As will be seen from the above, the development of 

 the buds in Pyrosoma proceeds according to the same laws 

 as those which govern gemmation in the Syiiascidiæ. The 

 outer vesicle has no share in the development of the liud. 

 but the most important organs are formed from the inner 

 vesicle. 



The fact that the periijranchial cavities and the ner- 

 vous system are not formed. as in the Synascidiæ. by a 

 simple evaginatiou of the wall of the inner vesicle, but that 

 onlv a small jjortion of the wall 1 miges out, is explained 

 by the great thickness of the cellular strata. 



As the cell w\alls retain their thickness throughout 

 the development of tiie bud, instead of being drawn out, 

 as in DistapUd. to low cylindrical-. or tiattened-epitheliuni. 

 the examination of PyrosaiiKi. is attended with much difti- 

 cnlty, for the walls of the different organs, especially in 

 the dorsal part of the hutl. at last lie elose up to one an- 



