1897.] Development of the Vertebral Column. 405 
the earliest rudiment of the vertebral centrum is a segment of 
the elastica externa, and that the cells of the skeletogenous tis- 
sue which develop into the intervertebral cartilages first make 
their way through the elastica. Whatever that earliest rudi- 
ment may be, it has nothing to do with the elastica externa. 
This is present, as it is in fishes, in close relation with the 
elastica interna. In the tail of young Necturus, where the 
bone is already well developed, the externa may be plainly 
seen as a highly refractive line between the interna and the 
centrum. But this is nothing new, since other observers have 
seen the externa distinct from the centrum. 
As regards the lizards and other Amniota, I am willing to 
concede that there intervenes between the bases of the arches 
and the sheath of the notochord a distinct cartilage on which 
rest the bases of the upper arches. Goette’s “ primary verte- 
bral centrum ” found in the lizards I regard as the pleurocen- 
trum, which has been pushed under the bases of the arches. [fit 
is such, we might expect to find it starting in its development 
on the upper side of the notochord ; and Goette’s figure shows 
at least that it is thinner on the lower side than elsewhere. I 
am ready to admit that in the Amniota the basalia, to use 
Gadow’s terms, have formed unions with the interbasals behind 
them, instead of with those in front of them. The possibility 
of this was considered in my former paper, p. 51. Conse- 
quently, in the vertebra figured by Goette Figs. 1, 2, 7, the 
centrum has been pushed forward under the arch in front of 
it. It is quite possible that its original myomeral position is 
no longer reproduced in the embryo. 
If our minds can once be freed from the idea of a primary 
centrum we shall probably find little reason for disagreement 
about the development of the vertebra in the different groups. 
It seems to me beyond doubt that the rhachitomous vertebre 
of the dorsal region of Eurycormus (Zittel, Handbuch, vol. 3, 
p. 230) must have been developed from the embolomerous 
vertebre of the tail. We have seen how the rhachitomous 
vertebree of the dorsal region of the young Amia unite to pro- 
duce the definitive vertebra. Thiollidre’s figures of Callopte- 
rus, reproduced by Goette, show us how, by the reduction of the 
