.46 FIELD COLUMBIAN MUSEUM ZOOLOGY, VOL. i. 



^composition of the vertebrae of our living amphibians, I am aware, 

 that I tread on uncertain ground, and that my conclusions may be 

 erroneous. On the other hand, some of the suggestions may have 

 some value. 



The origin and structure of the vertebrae of the amphibia have 

 been quite as much discussed as have the origin and structure of 

 those of the fishes; and there has been the same lack of unanimity 

 of opinion. Johannes Muller concluded (49 Neur., p. 73) that in the 

 majority of the members of the class the vertebral centrum arises as 

 .an ossified integral ring in the outer sheath of the notochord. In the 

 case of certain Anura (Pelobates, Pseudis, etc.), however, the centra 

 were found to arise wholly from the upper arches, and the notochord 

 took no part in the process. 



Among the Urodela Gegenbaur (29) recognized the existence 

 'Of two sheaths surrounding the notochord, an inner thicker, and an 

 outer thinner. XDutside of the latter is the skeletogenous layer, in 

 which, at intervals, are developed the cartilaginous upper arches. 

 The bases of the latter rest on the outer sheath of the notochord. 

 In the tail, lower arches possess similar relations. Midway between 

 the bases of the successive arches there is formed a ring of cells, which 

 later change to cartilage. There is no continuous cartilage surround- 

 ing the notochord. Ossification begins by the formation of a delicate 

 ring of bone immediately outside the outer sheath, and in contact 

 with the bases of the arches. Gegenbaur seems to imply that the ring 

 is complete from the beginning. Its edges extend forward and back- 

 ward until they approach those of the neighboring centra. While 

 this is progressing, the cartilaginous intervertebral rings have devel- 

 oped and have become enclosed each within the approximating ends 

 of two centra. In some cases the cartilaginous ring remains short 

 anterio-posteriorly ; in other cases it grows forward and backward 

 so as to extend from near the center of one vertebra to that of the 

 next behind. The arches at an early stage undergo ossification and 

 become continuous with the corresponding centra. We have then a 

 series of biconcave vertebras alternating with a series of cartilaginous 

 rings, which are enclosed in the concave ends of the vertebral centra. 

 Finally, each cartilaginous ring may become more or less segmented 

 transversely, one portion going to each of the two adjoining centra. 

 In the case of the frogs, Gegenbaur regarded the notochord as being 

 .surrounded by a continuous tube of cartilage from which spring the 

 upper arches and with which the intervertebral rings are also con- 

 nected. Ossification is introduced by the deposit of calcific matter in 

 the cartilage at the base of the upper arches. There is a center on 



