148 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
Introduction. 
CRITERIA OF SEGMENTATION. 
Morpnouogists 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 be composed of fused vertebra 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 direction. 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 
