On the Ancestral Form of the Chordata. 



A. 4. liV. ffubreclit. 



Professor of Zoology at the University of Utrecht. 



With Plate XXIII. 



An all important question was raised in biology when the 

 Law of Development came to be recognised as the only true 

 explanation of the facts as they lie before us. This was the 

 question : '^ From what invertebrate stock are the Vertebrata 

 evolved, and which amongst the Invertebrates at present living 

 approaches most closely in its organisation to this primitive 

 parent stock." In 1868 the solution appeared to have been 

 found when Kowalevsky's splendid researches concerning the 

 development both of Amphioxus and of the Ascidians could be 

 compared side by side. The Tunicate larva was for the time 

 being proclaimed to be the missing link^ to be of all Inverte- 

 brates the closest approach to the much-looked-for parent form. 



Since then the aspect of things has changed and later inves- 

 tigations, more especially those of Duhrn and of Ray Lankester, 

 have rendered it nearly certain that the Tunicata must, on 

 the contrary, be looked upon as degenerate Vertebrates which 

 can be hardly of much use in helping us to the failing clue. 



Dohrn, Semper, Hatschek, Leydig, Kleinenberg, and Eisig 

 are amongvSt the foremost who have suggested, and most 

 brilliantly expounded and argued, that the Annelids offer tlie 

 greatest number of points of resemblance with the Vertebrates ; 

 that they and the Arthropods have descended together with 

 the Vertebrata from a primitive type, distantly agreeing in 



VOL. XXIII. NEW SER. A A 



