11 



ved sammenligning med senen' stadier at være oprindelsen 

 til knoppens nervesystem. 



Fig. 9—15 at'l)ilder en snitserie af en knop paa dette 

 stadium, med snitretning lodret paa moderd.vrets hengdeakse 

 og altsaa parallel en linie a-b (fig. 8). 



Fig. 1 1 viser, hvorledes en del af den ene peribran- 

 chialcavitet afsnores for længer nede (fig. 10 og 9) at smelte 

 sammen med udbugtningen fr-a den anden, saa at de kom- 

 mer til at danne en aalien furhindelsc mellem hegge peri- 

 hranciiialcaviteter. 



Her kan ikke være tale om nogen forveksling med 

 anlægget til kloakken, da denne forst viser sig jjaa et langt 

 senere stadium (fig. 28 og 33); og man kan paa et og 

 samme snit gjennem et saadant stadium se baade kloakken 

 og nerverorets nederste del (fig. 37). 



Paa fig. 12, som viser et snit lige over det paa fig. 

 11 af hildede, ser man, at en del af peribranehialcavitetens 

 cellevreg fortykkes for længer oppe at vise sig som en selv- 

 sta^idig cellestreng (fig. 13 og 14). der dog ikke strækker 

 sig op over entoderniroret (paa fig. 15, et snit fra knoppens 

 øverste del, findes intet spor af nogen saadan streng). 



Fra nu af vokser knoppen forholdsvis mere i vertikal 

 retning, peribranchialcaviteterne hegynder i knoppens proxi- 

 raale del at udbrede sig nedover, og efterhvert som denne 

 udhredelse skrider frem, trænges ontodermrorets tarmanlæg 

 frem mod knopi)ens distale ende. Xogen væsentlig foran- 

 dring viser sig dog ikke for ved 



Stadium III. 



Her ser man (fig. 16) nervesijstemet paa et videre 

 udviklingstrin. Det nedre anlæg har ingen væsentlig foran- 

 dring undergaaet, men de to opstigende cellestrenge er 

 vokset sammen over entoderniroret, hvor de liar udviklet 

 sig til en stor cellehob. Ved snit gjennem denne celle- 

 masse ser man en mængde celler under deling, hvilket kan 

 forklare den luirtige udvikling af denne ovre del af nerve- 

 systemet. 



Fig. 17 og IS viser to snit af en serie, livis sniti-et- 

 ning er parallel linierne a-h og e-d paa fig. 16. Paa fig. 

 17 ser man, hvorledes nervesystemet ligger som en ring 

 ondcring entoderniroret og peril)ranchialcaviteterne. og hvor- 

 ledes dets nedre bue staar i aaben forliindelse med disse, 

 medens der endnu ikke er dannet iKjget hnlruni i de op- 

 stigende grene. Fig. 18 er et snit længer tbran, omtrent 

 ved c-d (fig. 16), og her ser man. at der har dannet sig 

 en spalte indvendig i den forreste del af celleholien. 



Nervesystemet v(jkser nu mindre end knoppens ovrige 

 del, og da denne fortsætter sin iioidetilvæxt, kommer hele 

 nervesystemet snart til at ligge paa knopi)ens dorsale side. 



and the ectoderm. Tliis double rudiment proves, on com- 

 j^arison with later stages, to be the origin of the nerroas 

 sysfé)!! of the Inid. 



Figs. 9 — 15 exliihit a series of sections from a Imd 

 in this stage, the jilane of tiie sections being perpendicular 

 to the long axis of the niother animal, and therefore paral- 

 lel to line a-h (fig. 8). 



Fig. 11 shows liow jiart of one peribranchial eavity 

 is separated, and nierges lower down (figs. 10 and 9) info 

 the evagination from the other, so as to form and open 

 coniiection lietweeu tiie two perihranehial cavities. 



There can l)e no possibility hoi-e of confounding this 

 witii the riuiimentai^y cioaca, as tids does not make its 

 a])pearance until a niuch later stage (jigs. 28 and 33); and 

 in one section of a bud in such a stage, hoth the cioaca 

 and the lower jiart of the nerve tube can he seen (fig. 37). 



In fig. 12, which shows a section just above that 

 shown in fig. 11, it will be seen that i)art of tlie cellular 

 wall of the jieribranchial eavity is thickened and aiipears 

 farther up as an independent cellular cord ifigs. 13 and 14) 

 wiiicii, however, does not extend upwards over the endo- 

 dermal tu])e. (In fig. 15, a section from the upper part 

 of the liud. tiiere is no trace of any such cord). 



Henceforward the liud grows comparatively more in 

 a vertical direction; the peribranchial cavities begin, in the 

 proximal part of tiie hud, to expand downwards, and si- 

 multaneously with the progress of this expansion, the intes- 

 tinal rudiment issuing from the endodermal tube is forced 

 towards tlie distal end of the bud. No essential change. 

 however, appears until the succeeding stage. 



Stage III. 



Fig. 16 shows the net-voii^ system in a more advanced 

 stage of development. The lower rudiment has undergone 

 no essential change, hut the two ascending cellular cords 

 have grown together over the endodermal tube, where they 

 have developed into a large cellular mass. On making 

 sections through this mass. a numher of cells are seen 

 in the proccss of dividing. a circumstance which may ac- 

 connt for the rapid develoiunent of this upper portion of 

 tlie nervous system. 



Figs. 17 and 18 show two sections from a series 

 whose plane is parallel with tlie lines a-h and e-d in fig. 

 1(). Fig. 17 shows how the nervous system lies in a ring 

 about the endodei-mal tube and the peribranchi.-il cavities, 

 and how its lower ai-cli is in open communication with 

 the l:itter, wliile no eavity is as yet formed in the ascen- 

 ding branches. Fig. 18 is a section tåken fartiier forward, 

 at about c-d ifig. 16). and liere it will be seen that a (deft 

 has heen formed internally in the front part of the cidlu- 

 lar mass. 



The nervous system now grows less rapidly th;in the 

 rest ot the bud. antl as the latter cimtinues to increase in 

 height, the whole nervous system in soon on the dorsal 

 side of the Inid. 



