316 
HENRY OUR. 
appears as iu fig. 26, H.C. Where this happens it is generally 
in the most anterior somites. 
Previous to the breaking open of the gill-clefts there appears 
on each side of the mandibular arch a small thickeniug and 
protrusion of the epiblast. These protuberances appear long 
before any of the external gills of the other arches. They 
become later rod-like structures, and are then easily recog- 
nised as the organs which Clarke has called “ balancers.” An 
examination of their structure and relations shows them to be 
homologous with the external gills. They are supplied with 
blood by the most anterior or mandibular fork of the ventral 
aorta, and a branch of the fifth nerve may be traced down to 
the neighbourhood of the base of each balancer. Fig. 25 shows 
a vertical section passing longitudinal to the axis of the embryo 
and nearly longitudinal to the balancer ( bl .). The balancer 
consists of a cylinder of compact epiblastic tissue, growing 
slightly thicker toward the distal end, where it forms a thick 
epiblastic cap similar to the epiblastic cap generally observed 
on the limb-rudiments of vertebrate embryos. Internally the 
balancer is nearly hollow, but is generally divided longitudinally 
through the greater part of its proximal length by a thin mem- 
branous network of which the function is probably to separate 
the courses of the arterial and venous blood. The large 
amount of blood which passes through the balancer indicates 
that it subserves in part a respiratory function. Balfour, 
following the account of Goette, has stated that the mandi- 
bular artery is never developed in Amphibians. In Amblystoma 
I find the mandibular artery developed, though in a less 
degree than the posterior branchial arteries. Owing to the 
early disappearance of the balancers this artery probably 
atrophies at an early date. Though my sections of Triton were 
not so favorable to the observation of this point, yet I find 
traces there of the existence of a mandibular artery. 
At a stage when the branchial clefts have broken through 
and the cartilaginous skeleton has appeared (fig. 30 h), the 
relation of the balancer to the mandible becomes even more 
pronounced. The quadrate cartilage sends out laterally a 
