312 THE PR^iORAL LOBE. 



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 myotomes 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 

 comparatively 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 



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 em- 

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

 p. 84 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 plausible 

 explanation on the supposition that Petromyzon is descended from a vertebrate stock 

 in which true branchial bars had not been evolved. 



