192 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



It is not possible to attach much weight to either of these arguments, 

 for slight changes in the position of organs are not unusual, and it is well 

 known that the ontogenetic acceleration or retardation in the relative 

 time of appearance of structures is by no means exceptional, and it 

 would be as safe to assume that the change in the pitch of the voice of 

 man is phylogenetically older than the sexual maturity of the ancestors 

 of man, as it is to assume, from the same sort of evidence, that the aortic 

 system of vertebrates is older than the mouth. 



The vertebrate mouth unquestionably bears a great morphological 

 resemblance to a pair of gill-slits. As Dohrn points out, it is bordered, 

 like the gill-slits, by a pair of visceral arches, it lies in front of the first 

 pair of true gill-slits, it arises at the same time with them in the embryo, 

 and like them it opens into a section of the gut. 



A ventral view of a shark shows the resemblance between the 

 mouth and the true gill-slits in the most impressive way, and if any 

 pair of them were to be united with each other at their ventral ends, 

 they would become perfectly equivalent to the mouth. The armature 

 of the mouth is repeated on the gills, and there is reason to believe that 

 the jaw-arches have at one time carried gills like the gill-arches. 



This resemblance is not imaginary. Beyond all question it is real, 

 and it is certainly most remarkable and suggestive, but does it prove 

 that the vertebrate mouth is phylogenetically a pair of gill-slits ? 



When, in my student days, my instructor held before me the skull 

 of a turtle and called upon me to observe the centrum, the transverse 

 processes and the neural arch of the occipital vertebra, I was, for the 

 time, convinced that the occipital bone had arisen by the differentiation 

 and specialization of a bony vertebra, like those in the neck of a turtle, 

 and that its history had been identical with that of the thoracic vertebrae, 

 which have been differentiated and specialized in the same way into con- 

 stituent parts of the bony box which covers the body of the turtle, as the 

 skull covers the brain. 



In all these cases the morphological resemblance is undeniable, but 

 our opinion of its phylogenetic significance depends upon our view of the 

 nature and origin of the metamerism of vertebrates, a question which 

 will soon be discussed. 



At present we must confine ourselves to a narrower point of view, 

 and learn where we are led by Dohrn's opinion that the vertebrate mouth 

 is actually a pair of gill-slits. 



If the present mouth of the vertebrates was once a pair of gill-slits, 



