312 EDWARD PHELPS ALLIS, JR. 
along the ventral surface of the brain, as that surface is defined by 
His; since as, when first formed, their hind ends apparently always 
lie anterior to the recessus infundibuli, they must themselves lie 
either definitely on the anterior surface of the brain or along 
the lateral surface of its extreme anterior end. In the latter 
case they would actually have, to the neural tube, the relations 
of dorsal vertebral arches. It does not, however, necessarily 
follow that they are such arches, for their relations to the brain 
may be wholly due to a cranial flexure so sharp and pronounced 
that it has turned the anterior surface of the neural tube down- 
ward upon cartilages which primarily lay either in the line of 
the axis of the body, or projected ventrally beneath it. 
SUMMARY 
A functional myodome is found only in fishes, and even among 
them it is limited, in-those I have examined, to Amia and the 
non-siluroid Teleostei. 
The myodome is always separated from the cavum cerebrale 
cranii by membrane (dura mater), cartilage, or bone, and the 
separating wall is in part spinal and in part prespinal in position. 
A depression in the prespinal portion lodges the hypophysis 
or both the hypophysis and saccus vasculosus, and this part of 
the wall never undergoes either chondrification or ossification, 
a more or less developed pituitary sac always projecting into 
the myodome. 
The myodome is found in its most complete form in the Tel- 
eostei, and there consists of dorsal and ventral compartments 
which are usually separated from each other only by membrane, 
but that membrane, the horizontal myodomic membrane, is 
capable of either chondrification or ossification. The dorsal com- 
partment lodges the hind ends of the musculi recti externi and 
is always traversed by a cross-c ommissural venous vessel formed 
by the pituitary veins. The ventral compartment lodges the 
hind ends of the musculi recti interni and is traversed by the 
internal carotid and efferent pseudobranchial arteries and the 
palatine branches of the facialis nerves. 
