43 



Disse to udviklingsstadier, der begge danner det ud- 

 gangsstadiuni, hvorfra organdannelsen begynder, viser utvivl- 

 somt en vis fornilighed. De bestaar begge af to epithel- 

 blrerer, af hvilke i beggo tilfælde den ydre danner dyrets 

 epidermis og den indre dyrets tarmtraktns. 



Ved en grundigere lietragtning viser det sig imidlertid, 

 at hermed ogsaa alle lighedspunkter er nævnt. 



Undersøger vi nemlig forst og fremst, hvm-ledes de 

 to udviklingsanlæg dannes, saa saa vi i kap. 1, ut gastrida- 

 stad/et frenigaar ved æggets gastrulation, og vi gjenkjender 

 i dets to blærer de to primære kimlilade, ektodermen og 

 entodermen, medens knopanlægget altid dannes af mere 

 eller mindre udviklede organer hos moderdyret. Knopan- 

 keggets ydre blære viste sig i kap. 2 altid at være en ud- 

 bugtning af moderdyrets hud, der ikke er identisk med 

 larvens ektoderm, fordi den er sterkt differentieret og for- 

 synet med et tykt celluloselag. Ikke mindre afviger knop- 

 pens indre blære fra larvens entoderm. Vi saa i kap. 2, 

 at den indre blære kan anlægges fra forskjellige organer, 

 hos Distcqjlia fra tarmtraktns,*) hos Amaroucluin fra epicar- 

 diet, hos Bofrijllns fra peribranchialsækken. End større 

 viser sig forskjellen at blive derved, at i)eribranchialsækken 

 hos larverne efter de .allerfleste forskeres samstemmige re- 

 sultater anlægges fra ektodermen hos larverne. Heraf drog 

 jeg nemlig den slutning, at kno[)anLæggets indre blære 

 saaledes oprindelig fremgaar af larvens ektoderm hos fami- 

 lien Boifijnidæ, og sammenligningen mellem knoppens indre 

 blære og gastrulastadiets entoderm bliver da — ud fra 

 kimbladlærens synspunkter — iallefald for BotrylUdernes 

 vedkommende en umulighed. 



Sammenligner man noiere de to stadiers hyr/nin//, da 

 tindes ogsaa her væscntlige forskjelligheder. Foruden at 

 knopanlæggets ydre blære som ovenfor nævnt er en lioit 

 differentieret epidermis med en celluloseschicht, saa er ogsaa 

 den indre blære meget forskjellig fra larvens entoderm. 

 Specielt bliver kontrasten stor hos de tamilier, der har 

 meget blonimerige æg. som f. eks. Disfaplia. hvor entoder- 

 men bestaar af store blommerisie celler, medens knoppens 

 indre blære er en liden lavcellet epithelmenibran. 



De foregaaende kapitler har nu ogsaa givet os mate- 

 rialet til at bevise, at de to udviklingsmodi under senere 

 stadier gjennemlober en vidt forskjellig ndviklingsproces. 



Under embryonaludvildingen fandt vi saaledes, at ga- 

 strulastadiets ydre blære, den primære ektoderm-, hos larven 

 danner følgende organer: 



1) Epidermis. 



2) Centralnervesystem, indbefattende larvehjerne, ganglion 

 og liypophysis. 



o) Peribranchialsæk. 



Derimod dannede gastrulastadiets indre blære, ..ento- 

 dermen- senere: 



These two stages of development, whieli each form 

 the starting-point for the formation of the organs, undoubt- 

 edly exhibit a eertain similarity of form. Tiiey each con- 

 sist of two epithelial vesiclos, the onter one in both cases 

 forming the epidermis, and the inner. the alimentary canal 

 of the animal. 



(_)n a closer consideratiou, it proves, however, that 

 the similarity ends hero. 



If we first of all emiuire into the manner in whicli 

 tlie two develop mental rudiments are Jo f med. we see, as 

 in Chapter I, tliat the gastrida stage is produced by the 

 gastrulation of the ovum, and we recognise in its two vesi- 

 cles the two primary germ-layers, the ectoderm and the 

 endoderm, while the bud-rudiment is always formed from 

 nioix' or less highly developed organs in the parent-unimal. 

 The outer vesicle of the rudimentary Imd was proved in 

 Chapter II to lie invarialily an evagination of the skin of 

 the parent animal, which is not identical with the ectoderm 

 of the larva, because it is highly difterentiated, and fur- 

 nished with a thick laver of cellulose. The inner vesicle 

 of the hud differs in )io less a degree from the endoderm 

 of the larva. We saw in ciiapter II that the iuner vesicle 

 can originate in vari us organs, in TJ/sfaplia, in the ali- 

 mentary canaT''!; in Ainaroiiciiiiti, in the epicardium: in Bo- 

 trgllus, in the jieribraiichial cavity. The differencc appears 

 still greater from tlie peribranchial cavity in the larva, 

 according to the unanimous results of nearly all naturalists, 

 originating in the ectoderm. From this I drew the con- 

 clusion that the inner vesicle of the bud-rudiment originally 

 proceeds from the ectoderm of the larva in the family 

 Botryllidu'. and the coraparison between the inner vesicle 

 of the bud and the endoderm of the gastrula stage then 

 becomes, from the germ-layer tlieory point of view, at any 

 rate as regards Boiryllidæ. an impossiliility. 



If we compare the structure of the two stages more 

 minutely, essential differences will also be found here. 

 Besides the outer vesicle of the Ijud rudiment being, as 

 belbre mentioned, a highly differentiated epidermis with a 

 cellulose laver, the inner vesicle is also very different from 

 the larva's endoderm. The contrast is especially great in 

 those families where the yolk of the ovum is very abun- 

 dant, as for instance in Disfaplii(. where the endoderm 

 consists of lai'ge, abundantly-yolked cells, while the inner 

 vesicle of the l)ud is a small, low-celled, epithelial membrane. 



The foregoing chapters have also given us material 

 to |)rove that the two ways of development, during later 

 stages, go tlirough very different developmental processes. 



Thus. during the embryonal derelopnieiif. we found 

 that tlie outer vesicle of the gastrula stage, „the primary 

 ectoderm", forms, in the larva, the following organs: 

 (1) The epidermis. 

 (2j The central organs of the nervous system, including 



the larval brain, ganglion & hypophysis. 

 (o) The peribranchial cavity. 



On the other hand, the inner vesicle of the gastrula 

 stage, the endoderm, subsequently forms: 



*) se note pag. 2 1 . 



*) See note, p. '21. 



G* 



