No. 470] 



UNITY OF GNATHOSTOME TYPE 



79 



taken together. They prove the essential unity and genetic 

 relationship of all the Vertebrata. 



Semper's view that amphioxus is not a true vertebrate has long 

 since been effectively disproved, and the most forceful part of 

 the proof that amphioxus does belong to the ancestral stock of 

 vertebrates is contained in its simple palingenetic development, 

 at least so far as the earlier stages are concerned. 



The method of laying down the gills up to the time that the 

 secondary gills are established, is in every way comparable with 

 the processes of gill formation in Bdellostoma. The transforma- 

 tion of the brain up to the apex of its development is similar to 

 the development of the vertebrate brain, the only difference being 

 that it stops its growth in a very primitive stage of the Craniate 

 brain. The differentiation of the tissues is in every respect the 

 same as that occurring in other vertebrates, except that the proc- 

 ess of tissue formation does not go as far, while some tissue sys- 

 tems, which appear in upper vertebrates, are never developed in 

 amphioxus. For example, the masses of connective tissues com- 

 mon to other vertebrates, in the form of true cartilage and bone, 

 are not even hinted at. The statement that its tissues are epithelial 

 is most erroneous. 



The lack of formation of such structures as jaws, chambered 

 heart, lacunar hepatic gland, cartilaginous skull, etc., is certainly 

 to be put down as a palingenetic characteristic. 



When we seek the type form of the vertebrate stock we are 

 forced to look to the invertebrates as the source of origin. Almost 

 all the groups of the Metazoa have been searched for the ancestral 

 type, and in nearly every case a type-form has been discovered 

 which shows the means of descent sufficiently satisfactory to the 

 individual zoologist to warrant a long and careful discussion of the 

 manner in which the morphologic and physiologic changes have 

 come about that have 'resulted in producing the vertebrates as 

 we find them to-day. 



Three types of structure have, however, l>een used more fre- 

 quently than the others. They are, respectively, nemertean, 

 annelid, and arthropod. All three of these types possess a suf- 

 ficient mmiber of cliaracteristics in connnon with the vertebrate 

 type to warrant many parallels being drawn between each of them 

 and the vertebrates. 



