ON THE ANCESTRAL FORM OF THE CHORD AT A. 1>.VJ 



;ui attempt to deal with the whole subject ; while the origin of certain 

 of the organs is dealt with in a more special manner in the chapters 

 on organogeny which form the second part of this work. 



Before entering upon the more special subject of this chapter, 

 it will be convenient to clear the ground by insisting on a few 

 morphological conclusions to be drawn from the study of Amphioxus, 

 a form which, although probably in some respects degenerate, is 

 nevertheless capable of furnishing on certain points very valuable 

 evidence. 



(1) In the first place it is clear from Amphioxus that the ancestors 

 of the Chordata were segmented, and that their mesoblast was divided 

 into myotornes which extended even into the region in front of the 

 mouth. The mesoblast of the greater part of what is called the head 

 in the Vertebrata proper was therefore segmented like that of the 

 trunk. 



(2) The only internal skeleton present was the unsegmented 

 notochord a fact which demonstrates that the skeleton is of com- 

 paratively little importance for the solution of a large number of 

 fundamental questions, as for example the point which has been 

 mooted recently as to whether gill-clefts existed at one time in front 

 of the present mouth; and for this reason: that from the evidence 

 of Amphioxus and the lower Vertebrata 1 it is clear that such clefts, if 

 they ever existed, had atrophied completely before the formation of 

 cartilaginous branchial bars ; so that any skeletal structures in front 

 of the mouth, which have been interpreted by morphologists as 

 branchial bars, can never have acted in supporting the walls of 

 branchial clefts. 



(3) The region which, in the Vertebrata, forms the oesophagus 

 and stomach, was, in the ancestors of the Chordata, perforated by gill- 

 clefts. This fact, which has been clearly pointed out by Gegenbaur, 

 is demonstrated by the arrangement of the gill-clefts in Amphioxus, 



1 The greater part of the branchial skeleton of Petromyzon appears clearly to 

 belong to an extra-branchial system much more superficially situated than the true 

 branchial bars of the higher forms. At the same time there is no doubt that certain 

 parts of the skeleton of the adult Lamprey have, as pointed out by Huxley, striking 

 points of resemblance to parts of a true mandibular and hyoid arches. Further ern- 

 bryological evidence is required on the subject, but the statements on this head on 

 p. 69 ought to be qualified. 



Should Huxley's views on this subject be finally proved correct, it is probable that, 

 taking into consideration the resemblance of these skeletal parts in the Tadpole to 

 those in the Lamprey, the cartilaginous mandibular bar, before being in any way 

 modified to form true jaws, became secondarily adapted to support a suctorial mouth, 

 and that it subsequently became converted into the true jaws. Thus the evolution of 

 this bar in the Frog would be a true repetition of the ancestral history, while its 

 ontogeny in Elasmobranchii and other types would be much abbreviated. For a fuller 

 statement on this point I must refer the reader to the chapter on the skull. 



It is difficult to believe that the posterior branchial bars could have coexisted 

 with such a highly developed branchial skeleton as that in Petromyzon, so that the 

 absence of the posterior branchial bars in Petromyzon receives by far its most 

 plans- ible explanation on the supposition that Petromyzon is descended fiom a ver- 

 tebrate stock iu which true branchial bars had not been evolved. 



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