148 BULLETIN: MUSEUAI OF COMPARATIVE ZOOLOGY. 



Introduction. 



Criteria of Segmentation. 



MoRPHOLOGiSTS have long sought to compare in Vertebrates a head 

 segment with a trunk segment. They have assumed that in the ances- 

 tors of Vertebrates head and trunk were differentiated from each other, 

 and that similar segments once extended throughout the entire length 

 of the body. Direct evidence in favor of this assumption is now fur- 

 nished, it is held, by Amphioxus. Because of the many difficulties in- 

 volved, the problem has become a favorite one, and since the early 

 attempts made by the poet Goethe and anatomists of the " Transcen- 

 dental" school, many men have contributed evidence and theory in the 

 hope of its solution. Since Goethe and Oken maintained the bony 

 cranium to he composed of fused vertebrae comparable with those in the 

 vertebral column, the problem has passed through several phases. First, 

 Huxley ('58), upon broad comparative anatomical evidence, proved that 

 nothing like a vertebra is to be found in the cranium of either high 

 or low Vertebrates, and he concluded as a result of his researches that 

 morphologists, in attempting to find a primitive metamerism in a struc- 

 ture which is so late in its phylogenetic appearance as the bony cranium, 

 were approaching the problem in the wrong du-ection. In thus dis- 

 proving the " vertebral theory " of the Vertebrate cranium, however, 

 " war die Frage doch noch nicht aus der Welt geschafft," as Gegenbaur 

 wrote in his famous "Kritik." By Gegenbaur ('72) the question was trans- 

 formed into a problem of the phylogenesis of the entire head. By using 

 as criteria the visceral arches and the nerves which innervate them, he 

 attempted to determine the number of primitive segments in the head 

 of those low Vertebrates, the Selachii, which in his opinion most re- 

 sembled the hypothetical Vertebrate ancestors. 



With the gradual acceptance of the " fundamental law of biogenesis," 

 that the development of an individual is an epitome of the develop- 

 ment of the race, the evidence offered in the solution of the problem 

 of the morphology of the Vertebrate head has become more and more 

 embryological. 



After Balfour's ('78) discovery that the primary body cavity of Sela- 

 chian embryos extends unbroken into the head region, and the further 

 discovery of Marshall ('81) that in these embryos the body cavity of the 

 head undergoes an independent segmentation into mesodermal cavities, 

 Selachian embryos became the chief objects of research. It was finally 



