328 THE RECAPITULATION THEORY 



of cells enclosed by the epiblast cells. It is in con- 

 nection with these variations that controversy has 

 arisen as to the primitive mode of development of 

 the gastrula, a point to which I shall return later 

 on. 



Among the higher Metazoa or Ccelomata the 

 extraordinary modifications in the position and in 

 every conceivable detail of formation of the meso- 

 blast in different and often in closely allied forms 

 have given rise to ardent discussion, and have led 

 to the proposal of theory after theory, each rejected 

 in turn as only affording a partial explanation, and 

 now culminating in Kleinenberg's protest against 

 the use of the term mesoblast at all, at any rate in 

 a sense implying any possibility of comparison with 

 the primary layers, epiblast and hypoblast, of 

 Coelenterata. 



This is not the place to attempt to decide so 

 difficult and technical a point, even were 1 capable 

 of so doing f but we may well take warning from 

 this extraordinary diversity of development, the 

 full extent ~oT~Wh~ich I believe we as yet realise 

 most imperfectly, that in our attempts to recon- 

 struct ancestral history from ontogenetic develop- 

 ment we have taken ^n hand no light task. To 

 reconstruct Latin from modern European languages 

 would in comparison be but child's play. 



Of the readiness with which special develop- 

 mental characters are acquired by allied animals 

 the brothers Sarasin* have given us evidence in 



* P. and F. Sarasin, "Ergebnisse naturwissenschaftlicher 

 Forschungen auf Ceylon," vol. ii. chap. i. pp. 24-38. 



