51 



i dyreriget or en iilentisk dannelse, idet det selv dannes 

 paa en forskjellig nuuide. Jeg henviser her til E. B. Wil- 

 sons iirægtige afhandhng: ,,Tiie cell-lineage of Nereis" (G8) 

 hvor han, pag. 367, preciserer et saadant standpunkt: ..It 

 appeai's to me, that the only course open to enibryological 

 investigation is to exaniine more precisely tiie origin of tiie 

 gastrula itself; to take as a sfcnihirj-point uot the fwo-layered 

 f/astrtda hut the ovuni. Tiie ..gastrula-' cannot he tåken as 

 a starting-point for tiie investigation of comparative organo- 

 geny nnless wc are certain tiiat tlie two layers are every- 

 wherc homologous. Simple to assume tliis homology is 

 simply to heg the question. Th; relat/ons]iip of the inner 

 and Older layers hi fhc varimis forms of gastrulas must be 

 iuvestigated not only hy determiiting tJieir relationship to 

 the adidt body, bnt also hy trac/ny ord the ceU-lineaye or 

 cytogeny of the individual blastomeres from tlie heginning 

 of develo2)ment.'^ 



Paa den anden side har der liævet sig stemmer, der 

 har iiævdet, at der gives tilfælde, i iivilke organerne dannes 

 andeiicdes end almindeligt. Saaledes har Heymous (30) 

 paavist, at hele tarmtractus hos visse insekter dannes af 

 ektodermen, et tilfælde, som han har liavt den venligiied at 

 demonstrere for mig med de mest overbevisende præparater. 

 Fremdeles har man anført mod kimbladlæren (i den lietyd- 

 uing vi hidtil har brugt dette ord), at organerne i kno[)- 

 udviklingen ofte anlægges paa anden maade end ellers (saa- 

 ledes hos Ascidier, Bryozoer, visse 3Jedaser). 



Disse stridsspørgsmaal har nu fort Bracm (7) til at 

 fremsætte den thesis, at kimliladene som de sædvanlig for- 

 staaes ikke er brugelige for embryologisk forskning. Istedet- 

 for at man, som liidtil, definerer ordet entoderm med ud- 

 trykket „gastrula]arvens indre blad'', mener Brae/m, at man 

 maa opfatte kiml)ladene fra et fysiologisk synspunkt. Pag. 

 42 udtrykker forfatteren dette saaledes (7): ,. Wir sehen 

 also, dass die Gastraeatheorie am Ende niclits weiter be- 

 sagt, als was von Anfang an feststand, dass namlich die 

 Keimblatter gleichbedeutend, d. i. analog seien; wenn Haec- 

 kel dafiir das Wort ,Jiomolog" setzt, ein Ausdruck, der 

 soust die Gleichheit der Lage zu Ijezeichnen ptlegte, so ist 

 das eine Sache fiir sich, es ist nur ein Sprachgebraucii.^' 

 Til denne opfatning synes specielt studiet af de afvigende 

 forhold hos kuopperne at have fort forfatteren. 



Det forekommer mig, at den morpliologiske og fysio- 

 logiske betragtning af udviklingen ikke er to synspunkter, 

 der udelukker hinanden, men tvertiraod supplerer og støtter 

 hinaiiden og begge er nødvendige for en biologisk forstaaelse. 

 En morphologisk betragtning af udviklingen maa have til 

 maal at studere de formdannelser og formforandringer, en 

 organisme gjeiinemlober fra regget og til det voksne individ 

 og at sammenligne de forskjellige typer med liinanden. 

 Brodrene Hertwig giver denne tanke folgende klare form 

 (2G, pag. 1): ,.Wenn die vergleichende Entwicklungsge- 

 schichte das reichliche aus zahllosen Einzeluntersuciiungen 

 ihr zustroniende Material wissenschaftlich verwerthen soll, 



that the gastrula stage is not an identical formation tiirough- 

 out the animal kingdom, it being itself formed in a dif- 

 ferent way. I would here refer to E. B. Wilsons excel- 

 lent treatise, the „Cell-lineage of Nereis" (G8), in which 

 (p. 367) he precisely defines such a stand-point: ,,It ap- 

 pears to me, that the only course open to enibryological 

 investigation is to exaininc more precisely the origin of the 

 gastrula itself; to tala; as a starting-point, not tla; two- 

 layercd gastrula. Imt the ovuni. The ,,gastrula'' cannot be 

 tåken as a starting-point for the investigation of compara- 

 tive organogeny. unless we are certain that tiie two layers 

 are cverywhere honiologous. Simply to assume this homo- 

 logy is simply to heg the question. Tlie relationship of 

 the inner and outer layers in the various forms of gastrnlas, 

 must he investigated, not only by determining their relation- 

 ship to the adult body, hut also hy tracing out the cell-lineage, 

 or cytogeny, of tiie individual hlastoiiures from tlie hegin- 

 ning of dereJopnu nt." 



On the other hand, voices iiave been raised, asserting 

 that there are cases in wliich the organs are formed otlier- 

 wise than in the ordinary way. Thus Heymous (30) has 

 shown that the whole alimontary canal in certain insects 

 is formed from the ectoderra, a circumstance which he has 

 been kind enough to demonstrato to me by means of the 

 most convincing preparations. Furthermore, it has been 

 quoted against the gerra-layer theory (in the sense in which 

 we have iiitlierto used the word), that in the Ijud develop- 

 ment tlie organs often coinmence in an unusual way (e. g. 

 in Ascidiæ, Bryozoa and certain Medusa'). 



These debated points have led Bruem (7) to put for- 

 ward the thesis, that the gei-m-layers in their ordinary ac- 

 ceptance, are of no use in embryological research, Instead 

 of. as hitlierto, defining the word endoderiu by the expres- 

 sion ,.the inner vesicle of tlie gastrula larva-', i*;-a''»i thinks 

 that the germ-layers ougiit to be regarded from a physio- 

 logical point of view. On p. 42 (7) the author thus ex- 

 presses tiiis: „Wir sehen also, dass die Gastraeatheorie 

 am Ende nichts weiter besagt, als was von Anfang an fest- 

 stand, dass namlich die Keimlilatter gleichbedeutend, d. i. 

 analog seien; wenn Haeclæl dafiir das Wort ..homolog-' 

 setzt, ein Ausdruck, der sonst die Gleichheit der Lage zu 

 bezeichnon pflegte, so ist das eine Sache fiir sich, es ist 

 nur ein Sprachgebrauch." The study of the dififering con- 

 ditions 111 the buds, seenis especially to have led the author 

 to take this view. 



It appears to me tiiat tiie morphological and the 

 plivsiological considerations of the development are not two 

 points of view which exclude one anothev, bnt on the 

 contrary, they supplement and support one another, and 

 are both necessary to a biological comprehension. The 

 aiin of a morphological consideration of the development 

 must lie to study the formation and changes of form through 

 wliicii an organism may pass, from the ovum to the full- 

 growii animal, and to compare the various types witli one 

 another. Tiie brothers Herttvig give tiiis thought the follow- 

 iiig clear expression (26, p. 1^: ,,Wenn die vergleichende Ent- 

 wicklungsgeschichte das reichliche aus zahllosen 'Einzelunter- 



