1833.] tn the cold-blooded Vertebrata. 471 
bre of the tail are gradually absorbed ; enlargements of the cord are 
evident at the points where the organs of locomotion are to be pro- 
duced, and shortly after these organs begin to display themselves. 
At the same time an important change takes place in the relative 
magnitude of the hemispheres of the brain and the optic lobes. In 
fish, as before stated, the optic lobes are the larger, the hemispheres 
having attained but a very inferior degree of development ; during the 
metamorphosis of the larva, the latter rapidly increase in size, till they 
have become considerably larger than the optic lobes; the olfactory 
tubercles are no longer separated from the hemispheres ; the whole 
cerebral mass having assumed a more concentrated form, instead of 
presenting the appearance of a number of imperfectly united ganglia. 
In speaking of the mode in which the blood circulates in the larva, 
I described four branchial arteries on each side, passing to as many 
respiratory organs, and conveying to them the blood which is to 
undergo the process of oxygenization. 
Synchronously with the change which the nervous system undergoes, 
this mode of circulation experiences considerable and important alter- 
ations ;—the anterior branchial arteries, which are so many subdivi- 
sions of the aorta, are obliterated—the posterior branchial artery alone 
remaining pervious; while its numerous ramifications are reduced to a 
single trunk, the union of which with the artery of the opposite side 
forms the trunk of the abdominal aorta. From the thoracic aorta is 
given off on each side a small pulmonary twig, which now becomes 
the channel through which the blood passes to the organs of aeration. 
Another remarkable circumstance is the change which now takes 
place in the form of the vertebre. The vertebra of a fish, we have said, 
presents two cup-like articular surfaces, the space intermediate between 
two vertebre being filled up by elastic cartilage. The vertebre of 
reptiles always present one convex and own concave articular surface, 
the globular head of one vertebre fitting into the concavity of the one 
immediately below it, so as to form a ball and socket joint. While in 
the tadpole state, the vertebrae of the animal resemble those of fish ; 
but it was observed by Durrocuet, that, at the period when the change 
in the respiratory apparatus was going on, the intervertebral substances 
became ossified, each uniting itself to the vertebra immediately pre- 
ceding. 
When the metamorphosis has been fully accomplished, the lungs of 
the adult animal are found to be tolerably perfect, but still not so 
minutely cellular, and consequently not presenting so extensive a sur- 
face for the aeration of the blood as in the higher reptiles. Never- 
