10 



ror, umler. over og pau siderne af entoilfrmroret. Det 

 første af disse optages af niesoderrastrengeii. over entoderni- 

 roret findes paa dette stadium endiui kun spredte meso- 

 dermceller, medens hegge siderør oi)tages af peribrcuicJiicd- 

 rørene (fig. 6). 



Paa dette stadium er perihranchialcariteteriie altsaa 

 anlagte, og ved at betragte en snitserie (fig. 1 — G) vil man 

 se, hvorledes de dannes ved at en del af entodermroi'ets 

 cellevæg paa hver side bugter sig ud for at lægge sig som 

 et lilad ved siden af dette. Snitretningen betegnes ved 

 linierne a-h, e-d, o. s. v. paa fig. 7. og saaledes, at fig. 1 

 betegner et snit gjenneni linien a-h, fig. 2 og 3 er to jiaa 

 hinanden følgende snit ved r-d. fig. 4's stilling betegnes 

 ved e-f, 0. s. v. 



Perihrancliialcaviteternc anhegges vistnok noget tid- 

 ligere, paa et stadium svarende til Seel/r/ers fig. 5 (2); men 

 jeg har til min fremstilling valgt en saa udviklet knop, for 

 at jeg ved afbildninger som fig. o og (j skulde være istand 

 til at fjerne enhver tvil om, at det virkelig er peribranchi.al- 

 caviteterne. som anlægges ved denne udbugtning af ento- 

 dermen. 



Paa noget yngre knopper har jeg ved at benytte en 

 anden snitretning fundet billeder som fuldstændig svarer til 

 tSeellffr)-^ fig. 41 og 43 (2); men som man vil se af fig. 7, 

 kan man ikke faa noget tydeligt billede af sammenhængen 

 mellem entodermrøi-et og periln-anciualcaviteterne uden netop 

 ved serier, som har nogenlunde samme snitretning som den, 

 jeg har afbildet. 



Eftersom knopjjen nu vokser, afsnøres dens foriiin- 

 delse med moderdyret noget. saa den staar fæstet ved en 

 tyndere stilk, hvori dog endnu alle knoi)pens organer findes. 

 Af foi-andringer, som foregaar i knoppens indre, kan merkes: 



enf oder mr ørets to nederste grene vokser sterkere end 

 de øverste, og danner derved det forsto anlæg til tarm- 

 tractus; 



i pfrihrancltiahariteierne. som stra^kker sig helt ud 

 mod knoppens spids, optranler et tydeligt hulrum; 



mesodermstrengeii , hvori knoppens æg træder tydelig 

 frem mod de øvrige celler, strækker sig forover omkring 

 entodermrørets spids, idet den lægger sig tæt op til ekto- 

 dermen, hvor den holder sig som et sammenhængende lag 

 giennem de foliiende stadier. 



Stadium II. 



Ved at Ijetragte fig. 8. som er en halv schematiseret 

 fremstilling af en kno|) paa dette stadium, ser man, at 

 begge perihranclikdcavifcter i knopi)ens dorsale parti, det, 

 som ligger nærmest moderdyret, har forenet sig under eu- 

 todermrøret; samtidig ser man paa hver side et tapformet 

 anlæg, som udgaar fra peribranchialcaviteten lige over for- 

 bindelsesstedet, og som strækker sig et stykke opover mel- 

 lem denne og ektodermen. Dette dobbelte anlæg viser sig 



ectoderm and the endoderm. fuur parallid tubes, respectively 

 beneath. aliove. and at the sides of the endodernial tube. 

 The first of these is occupied by the mesoderni cord; above 

 the endodermal tube at this stage, there are as yet only 

 scattered mesoderm cells, while the two lateral tubes are 

 occui-Hed by the perihrancliiol tuhes (fig. 6). 



Thus the formation of the perlhraiidi/al rav/ties has 

 commenced at this stage, and on looking at a sectional 

 series (figs. 1 — 6). it will l)e se en that they are formed liy 

 the evagination of a portion of the cellular wall on each 

 side of the endodermal tube. beside which they lie like a 

 leaf. The plane of the sections is iudicated by the lines 

 a h. r (1. etc. in fig. 7. Thus. fig. 1 n-presents a section 

 through line « h, figs. 2 and 3 are two consecutive sections 

 at c d. the plane of fig. 4 is iudicated by the line ef, etc. 



The perihranchial car/fles commence, it is true. rather 

 earlier, at a stage answering to Seellgvfs fig. h (2); Init 1 

 have chosen a bud of the more advanced stage, in order 

 that I might. by such figures as .5 and 6. remove all doubt 

 as to the really being the perihranchial cavities which 

 commence with this evagination of the endodei'm. 



By taking sections in another })lane. I iiave (jbtained, 

 in a soniewhat less advanced bud. results which exactly 

 correspoiul with Seel/f/er's fi.sis. 41 and 43 (2); but as my 

 fig. 7 shows, no very clear idea of the connectiou between 

 the endodermal tube aud the peribranchial cavities is to 

 be gained, except by such series as are in a ]ilane sonie- 

 what similar to that which 1 have here given. 



With the growtli of the bud. its connectiou With the 

 mother animal becomes nuire iiiter)n]:)ted, until it is atta- 

 ched liy a thiii stalk, in which, howevei-. all the organs are 

 still fouiid. Among the clianges which take place in the 

 interior of the Imd, the following may be noticed: 



The two lowest branches of the endodermal tuhe grow 

 more quickly than the njjper ones. and thus form the first 

 i-ndinuMits of the intestinal region; 



A distinct cavity appears in the perihrancliiaJ fahes. 

 which exteud to the very point of the bud; 



The mesoderniaJ eord, in which the ova of the luicl 

 are cønspicuous among the other cells. extends forwards 

 about the point of the endodermal tube lying close up to 

 the ectoderm, where it remains, like a compact stratum, 

 through the subsequent stages. 



Stage II. 



Un looking at fig. 8, which is a semi-diagrammatic 

 representation ot a Ijud in this stage, it will be seen that 

 the two iierihrauchial c((riti('s in the dorsal part of the l)ud 

 — the part nearest to the mother animal — , have become 

 united Ijeneath the endodernial tube. At the same time 

 there is visible, on each side. a i)rocess-like formation, issu- 

 iug from the i)eribranchial cavity just above the point of 

 union, and extending a littlc way up between the cavity 



