128 THE ORIGIN OF VERTEBRATES 
bars of other higher fishes, in that it forms a system of cartilages 
which lie external to the branchize—an extra-branchial system. 
This branchial basket-work is simpler in Ammoccetes than in 
Petromyzon, and its actual starting-point consists of a main trans- 
— verse bar corresponding to each branchial 
segment; from this transverse bar the 
system of longitudinal bars by which 
the basket-work is formed has sprung. 
These transverse bars arise from a 
cartilaginous longitudinal rod, situated 
close against the notochord on each 
side. These rods may be called the 
subchordal cartilaginous bands (Fig. 53), 
and, according to the observations of 
Schneider and others, each subchordal 
band does not form at first a continuous 
cartilaginous rod, but the cartilage is 
conspicuous only at the places where 
the transverse bars arise. In the 
youngest Ammocetes examined by 
Schaffer, he could find no absolute dis- 
continuity of the cartilage except be- 
tween the first two transverse bars, but 
he says that the thinning between the 
transverse bars was so marked as to 
make it highly probable that at an 
earlier stage there was discontinuity. 
The whole systém of branchial bars and 
subchordal rods is at first absolutely 
ase disconnected from the cranial system of 
Fic. 54—Ventran View OF trabecule and parachordals, and only 
Huan Raion or AMMOC@NES. tater do the two systems join. 
ahs on Grae en These observations on Ammoccetes 
lead most definitely to the conclusion 
that the starting-point of the whole cartilaginous skeleton of 
the vertebrate consisted of a series of transverse cartilaginous bars, 
for the purpose of supporting branchial segments; these were con- 
nected with two axial longitudinal cartilaginous rods, which at first 
contained cartilage only near the places of junction of the branchial 
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