MYODOME AND TRIGEMINO-FACIALIS CHAMBER 247 
externus is shown lying between the parasphenoid and the plate 
of parachordal cartilage, and extending posteriorly beyond the 
prootic region into the anterior end of the basioccipital region. 
The plate of parachordal cartilage now forms a prootic bridge, 
but how it has been developed is not explained. In the base 
of the median process of the parasphenoid is a block of cartilage 
said to represent the anterior end of the parachordal, the median 
process of the paraphenoid thus lying posterior to the fenestra 
hypophyseos. The pituitary body (hypophysis) and infundib- 
ulum (saccus vasculosus, both shown lying posterior to the 
median process of the parasphenoid, must then also lie poste- 
rior to the fenestra hypophyseos, and this is apparently also 
their position in advanced embryos and the adults of certain 
other, if not all fishes, as will appear later. 
Swinnerton does not describe the internal carotid arteries, 
but it seems certain, both from his figures and from the condi- 
tions in a 40-mm. specimen of this fish, described immediately 
below, that these arteries pass upward between the hind ends 
of the trabeculae and that they are never there enclosed in 
cartilage. 
In the earlier stages considered by Swinnerton the rectus 
externus muscles are said to be inserted into each other and 
into the tissues filling the hind part of the fenestra interpara- 
chordalis. In the third and fourth stages they extend poste- 
riorly so that their hind ends lie beneath the posterior border 
of that fenestra, and hence along the ventral surface of the basis 
cranii. The eyeballs have in the mean time descended to a 
level relatively lower than in the earlier stages, and this is said 
to cause the eye-muscles to press upon the anterior prolonga- 
tions of the parachordals and, depressing them, institute the 
beginning of the formation of the myodome. It would nat- 
urally be supposed that this depression would affect the hind 
ends of the trabeculae, with which the parachordals are fused, 
and this is what actually takes place in Salmo, as described by 
Gaupp and to be later considered. In Gasterosteus, on the con- 
trary, the hind ends of the trabeculae have not been in the least 
depressed in the oldest stages shown by Swinnerton in which 
