( 844 ) 



To look upon tlioiii as potenlial iiiotlier-cells of lliose iTies<»l»last-ltaH(ls 

 would he against all kjiown laws of horeditv, where in all othei- points 

 there is so great a resemhlance, also with regard to the niesenchyni, 

 hetween this 64-eellnlar stage and that of Annelids and Molluscs and 

 where it would he entirely inipossihle — supposing evolution to have 

 followed the line : Coelenlerates, C'tenophora, Plathelniinths — to 

 derive the niesoblast-bands, which must auyhoAv lie accumulated in 

 the cells mentioned, from these precedijig ancestral forms. On the 

 other hand it can easily be understood that these bands have gradually 

 assumed their present form and peculiar characteristics in the long 

 (and to us nidvnown) series of the ancestral forms of Annelids, 

 Molluscs and Polyclada, and that with these latter and still more 

 with the Ctenophora- (which have an ontogenesis so much resembling 

 that of the Polyclada,) the part })layed by these mesoblastic mother-cells 

 has again receded to the back-ground. 



We must then, especially on account of what ontogenesis has 

 taught us, consider the Plathelniinths as degenerate descendants of 

 Coelomata and so the strobilation of the Cestoda, which are still 

 further degenerated by parasitism, again fails within the reach of an 

 explanation which would homologize it with the metameric structure 

 of the Annelids. 



How the gradual I'eduction leading from Polychaeta via Oligochaela 

 and Hirudinea to Plathelniinths, has left its traces in all the diiferent 

 organs and tissues I will not develop more extensively here; I may 

 suppose these points to be generall}' known. 



It is obvious, after what has preceded, that ue ought not to attempt 

 to deri\'e the metamerism of the Annelids from the })sendo-metanierism 

 of the Turbellaria as Lang does. I prefer to accept the hypothesis 

 formulated already in 1881 by Sedgwick, according to Avhich a longi- 

 tudinally extended, actinia-like being, possessing wormlike free motility, 

 formed the starting-point. Gradually cyclomerism Avas converted into 

 bilateral symmetry and linear metamerism, in tlie same way as now 

 already certain Actinia show a tendency to bilateral symmetry. 



Ed. van Beneden afterAvards indicated (1894), though only in an 

 oral adress which has never been published, how Sedgwick's A'iew 

 might be extended to the Chordates. In 1902 in the "Verhandelingen" 

 of this Academy, I have tested the possibility of a[tplying Sedgwick's 

 theory to the facts that are i-evealed to us by the development of 

 mammals. And the facility Avith Avhich the explanation of Sedgwick 

 can be extended both to Vertebrates and Invertebrates, is undoubtedly 

 an argument in its favour. 



Lang and Hatsciiek object thai a prolonged actinialike being would 



