402 The American Naturalist. [May, 
thing very different from what it is in the other groups of ver- 
tebrates. 
Considering that the limits of the “äussere Chordascheide,” 
its significance, and even its existence, are not yet agreed upon, 
the alleged product of its transformation, the “ primary verte- 
bral body” is a thing too intangible to be long considered in 
the presence of manifest realities. 
Goette claims that in the tail of Amia the two disks belong- 
ing to each somite are two nearly equally developed vertebre ; 
it happening that only the arches of one of them are in a 
vestigial condition. I do not betieve that the elements which 
give rise to the two rings are equivalent. The arches, basi- 
dorsals and basiventrals of Gadow, arise at a later period than 
do the intercalated cartilages; at least they do so in Amia. 
The arches occupy a position essentially different from that of 
the intercalated elements, the former being placed intermyo- 
merically, the latter myomerically. The arches have prob- 
ably been evolved as a means for the fixed attachment for the 
muscular segments: the intercalated cartilages as a system of 
stop-gaps. Later these subordinate pieces have in many cases 
assumed a more important role. 
I have been able to discover no reason for supposing that 
there is, in each somite of Amia, either one or two “ primary 
vertebral centra.” For, if by “ perichordal sheath” Goette 
possibly refers to the elastica externa, this in Amia is certainly 
homologous with the structure so-called in the Teleost fishes 
and which, according to ail recent observers, is not cellular. 
If by perichordal sheath Goette means a distinct layer of cells, 
which lies against the outside of the elastica externa and 
passes beneath the arches, then there is no such sheath. In 
my smallest specimens, 10 mm. long, a delicate layer of cells 
surrounds the notochord, but it does not show itself as a special 
layer under the bases of the arches, although the latter are not 
yet distinctly chondrified. Nor is there at any stage any such 
a well defined layer of tissue under the cartilages. Conversion 
of the arches into hyaline cartilage begins at a little distance 
away from the elastica, but when the process is completed, the 
cartilage comes into immediate contact with the elastica. Nor 
