HUXLEY, ON THE SKELETON OF FISHES. 39 
Consequently, Vogt’s observations did not prove in the 
least that a truly homocercal fish ever passed through a 
heterocercal state; and as no observations respecting the 
development of the tail in what were supposed to be truly 
homocercal fish were extant, the doctrine that heterocercality 
precedes homocercality in embryonic life, was clearly not 
proven. On the other hand, until the development of some 
admitted homocercal fish had been examined, it was not 
disproved. 
Having procured a number of very young sticklebacks 
and eels, which would assuredly be admitted to be true 
homocercal Teleostei, if such things exist at all, I gladly 
availed myself of the opportunity of examining into this 
point. I was not a little surprised to discover, not only that 
these fishes are heterocercal in the embryonic state, but that 
they are perfectly heterocereal in the adult condition, their 
apparent homocercality being, as in the case of the salmon 
and its allies, a mere disguised heterocercality. 
In a Stickleback +;ths of an inch long (fig. 1), I found 
the gradually tapering extremity of the notochord (ce) bent 
upwards at a considerable angle with the axis of the body, 
and terminating close to the superior rounded corner of the 
caudal fin. In the greater part of its extent it was enclosed 
neither in cartilage nor in bone—though bony rings, the 
rudiments of the centra of the vertebre, were developed in 
the wall of the notochord throughout the rest of the body. 
The last of these rings (4) lay just where the notochord 
began to bend up. It was slightly longer than the bony 
ring which preceded it (a), and instead of having its posterior 
margin parallel with the anterior, it sloped from above, 
downwards and backwards. Two short osseous plates (e), 
attached to the anterior part of the inferior surface of the 
penultimate ring, or rudimentary vertebral centrum, passed 
downwards and a little backwards, and abutted against a 
slender elongated mass of cartilage (gy). Similar cartilagi- 
nous bodies occupy the same relation to corresponding plates 
of bone in the anterior vertebre in the region of the anal 
fin; and it is here seen, that while the bony plates coalesce 
and form the inferior arches of the caudal vertebree, the car- 
tilaginous elements at their extremities become the inter- 
hemal bones. The cartilage connected with the inferior 
arch of the penultimate centrum is therefore an “inter- 
hemal” cartilage. The anterior part of the inferior surface 
of the terminal ossification likewise has its osseous inferior 
arch (f), but the direction of this is nearly vertical, and 
though it is connected below with an element (4) which 
