59 



At udvikliiigen fonr.aar op til senere stadier at 1)e- 

 vare et imliffereiit materiale synes mig ogsaa at frenigaa 

 af de pathologiske misdannelser og af de saakaldte teratonier. 

 I visse sacraltnmorer kan man saaledes undertiden finde 

 alle vævsarter (bruskstykker. tverstrihet nuiskulatnr. sandse- 

 epithelier) repræsenterede. 



Ogsaa disse bekjendte forhold synes derfor at niaatte 

 henføres til udifferentieret materiale. 



Medens saaledes sporgsraaalet om, iiaar en specifika- 

 tion indtræder i udviklingen er et problem, der i hvert en- 

 kelt tilfælde maa loses ved empiriske undersogelser. saa er 

 det andet sporgsmaal nemlig, hvori specilikationens væsen 

 bestaar, hvad der betinger den. af meget mere theoretisk art. 



Vi saa ovenfor, at Boux og hans skole tænker sig, 

 at hver kjerne fra første oieblik er ..aktiveret" for dannel- 

 sen af en bestemt del af det voksne dyr (f eks. venstre 

 kropshalvdel), men at ved skadeligt indgreb, f. eks. adskil- 

 lelse af en furingskugle, regulerende krrefter. ,.regenerations- 

 plassonet" iidløses. 



Hertnig og Driesch tænker sig derimod saavel den 

 normale som den forandrede ndvikling afhængig af aarsa- 

 ger, der ligger udenfor anlægget selv; og som saadanne 

 aarsager har han nævnt cellernes gjensidige situs i forhold 

 til hinanden, saa udenverdenens kræfter (tryk, varme etc, etc). 



Det forekommer mig, at disse to theorier er to for- 

 skjellige iidtryk for den ene og samme ting. Begge theo- 

 rier søger nemlig i virkeligheden at forklare de forandringer 

 i en udviklings forlob, man eksperimentelt kan fremkalde, 

 som en organismes reaktion paa en „Reiz". Betragter vi 

 Eoiix' theori, at en furingscelle oprindelig er ..aktiveret" 

 for dannelsen af en bestemt del (f. eks. høire kropsdel), 

 men at cellens „regenerationsplasson" udløses, naar cellen 

 isoleres, hvad udsiger da denne theori andet end, at furings- 

 kuglen, naar den udvikler sig under normale forhold (,.Reize") 

 danner høire kropsdel og under andre forliold (,,B,eize") 

 noget andet. Hvad er ..aktivering'' og „regenerationsplas- 

 son- andet end organismens forhold overfor normale og 

 anormale „Reize". Paa den anden side ijetoner saavel 

 Hertwig som Dr/e,scJ/, at de aldeles ikke tænker sig, at 

 udenverdenens kræfter virker paa organismen jjaa anden 

 maade end som ,,E,eiz-'. Har man imidlertid opnaaet enig- 

 hed herom, saa har man vel dermed naaet grændsen for 

 biologiske forklaringer. Man kan vel som biolog studere 

 organismens forliold overfor en bestemt ,.Eeiz-, men deri- 

 mod er vel selve reaktionens væsen ligesaa utilgjængelig for 

 biologisk forskning som percepticnien af sandseindtryk o. s. 

 v. Saadanne diskussioner som mn udviklingen skyldes ak- 

 tiverede indre kvaliteter eller en livskraft eller fysikalsk- 

 chemiske processer forekommer mig derfor at burde tilliore 

 andre videnskaber end biologien. 



I sine ,,Lei;ons sur les phénomenes de la vie" siger 

 Oaiide Brrnanl: ,.Lors(iue le physiologiste vondra connaitre, 



The fact that the develoinnent is able to preserve an 

 inditferent material until later stages, seenis to me also to 

 be shown by the patiiological mal-formations and from the 

 so-called teratomata. In certain sacral tumours all kinds 

 of tissue (cartilage, crøss-striped musculature, sensory epi- 

 tlielium) may thus sometimes be found represented. 



It woiild therefore seem as if these well-known cir- 

 cumstances must also be referred to unditferentiated mate- 

 rial. 



While the question as to iclten a specification coni- 

 niences in the development is a problem wliich in every 

 single case must be solved by empirical investigations. the 

 other great developmental-mechanical question, nanicly, in 

 what the nature of the specification consist, upon what it 

 is contingent, is of a inuch more theoretical nature. 



We have seen above that Iloux and his school ima- 

 gine each nucleus to be ,,actnated" from the very first for 

 the formation of a certain part of the full-grown animal 

 (e. g. the left half of the body), l)ut that by injury, e. g. 

 the separation of a segmentation sphere, regulating forces, 

 the „regenerationsplasson" is released. 



Hertwig and Driesch, on the other hand, consider both 

 the normal and the altered development to be dependent 

 upon causes outside the rudiment itself; and as such causes 

 they have mentioned the position of the cells in relation to 

 each other, and external forces (pressure, heat. etc, etc). 



It seenis to me that these two theories are different 

 expressions for the same thing; for they both in realitj' 

 attempt to exjilain the changes in the course of a deve- 

 lopment which can lie experimentally produced, as the 

 reaction of an organisni on a ,.Reiz". If we consider 

 Boux's tlieory, that a segmentation cell is originally actuated 

 for the formation of a particular part (e. g. the right hand 

 part of the body), but that the celVs „regenerationsplas- 

 smi  is released when the cell is isolated, what else does 

 this theory assert than that the segmentation sphere, when 

 developed under normal conditions („Reize-'). forms the 

 light hand portion of the body, and under other conditions 

 (..Reize"), sometiiing else? AVhat are „Aktivirung-' and 

 ..Regenerationsplasson" but the conditions of the organism 

 with regard to normal and abnorinal ..Reize"? On the 

 other liand liotii Herhcig and Driesch accentuate the fact 

 that they by no means consider that external forces affect 

 the organisni otherwise than as ,,Reiz". If, however, on 

 this point harraony is established, it may be presunied that 

 the limit for biological explanatioiis has been reached. As 

 liiologist. one may study the conditions of the organisni 

 with regard to a particular ,,Reiz", but on tlie other iiand 

 the real nature of the reaction is as inaccessible to biolo- 

 gical research as the perception of sense-im|)ressions, etc. 

 Such discussions as to whether the development is due to 

 actuated internal qualities, or to a vital force, or to physi- 

 cal-chemical processes, seems to me therefore. to helong 

 more properly to other branclies of science tlian to biology. 



Claude Bernard, in his «Le^ons sur les phénomenes 

 de la vie", says: „Lorsque le physiologiste voudi^a connaitre, 



