38 FIELD COLUMBIAN MUSEUM ZOOLOGY, VOL. i. 



In the middle tail region we find apparently two kinds of centra r 

 those with and those without arches. The latter, the so-called pleu- 

 rocentra, are formed wholly through the union of the ossifications 

 arising from the upper with those arising from the lower intercalated 

 cartilages. Of course the cartilages themselves continue to consti- 

 tute a part of such a centrum. On the other hand, the centra of this 

 region which are furnished with upper and lower arches are produced 

 through the fusion of the ossifications which arise at the bases of 

 these two arches. They contain nothing more ; unless, to be exact,, 

 we expressly mention the persisting cartilage and the enclosed por- 

 tion of the notochord and its sheaths. 



The posterior tail vertebrae are mostly like the vertebrae last de- 

 scribed. They are produced wholly through the union of the bases 

 of the upper and lower arches. We find, therefore, in the vertebral 

 column of Amia no support for the doctrine that the vertebra is com- 

 posed partly of a central portion which is distinct in origin from the 

 cortical portions, and which has its origin in one of the notochordal 

 sheaths. In Amia the whole vertebra originates from the ossifications- 

 which spread out from the bases of the arches or from the intercal- 

 ated cartilages, or from both. Clear evidences to the same effect are 

 given by Balfour and Parker's work on Lepisosteus. In that fish, too, 

 neither of the notochordal coverings furnished any portion of the 

 vertebral centrum ; all the bone developed from the cartilages sur- 

 rounding the notochord. Indeed, it is only where there are gaps in 

 the cartilaginous tube surrounding the notochord that the bone comes 

 at all in contact with the elastica, and evidently the bone did not orig- 

 inate where these gaps are. The notochord can therefore hardly be 

 truly said to take the active part in the development of the bony ver- 

 tebral axis that some authorities have attributed to it. 



Nevertheless, I am not prepared to say that the coverings of the 

 notochord never undergo ossification. In some of the more advanced 

 larvae of Amia itself, I find that the inner sheath, the cuticula chordae, 

 has every optical appearance of having become calcified, and it stains- 

 exactly like the basis of the bone outside of the sheaths. But I am 

 not sure that this has happened. Goette agrees with Kolliker that in 

 Anguilla the cuticula chordae undergoes partial ossification (33, p. 

 125). It is, however, the outer sheath, the elastica, which is regarded 

 by some writers as calcifying and furnishing the primitive ring of the 

 vertebra. It is quite certain that this elastica is a secretion product 

 of the innermost layer of the skeletogenous tissue. It would not 

 then be strange if the earliest film of bony substance poured out by 

 this skeletogenous tissue were to occupy the apposed elastica. But 



