256 EDWARD PHELPS ALLIS, JR. 
opposite sides are separated from each other by a fissure (Fis- 
sur) the posterior, interparachordal portion of which is en- 
tirely filled by the projecting anterior end of the notochord. 
There accordingly is, as described by Schleip, no interparachordal 
fenestra in these embryos. The intertrabecular portion of the 
fissure is called the pituitary fossa and it is said to be closed 
ventrally by the parasphenoid, which, at these stages, extends 
from the orbit only to the tip of the notochord (Il. c., p. 354). 
The rectus muscles, in running from their insertions on the 
bulbus to their points of origin, are said to lie, in their ante- 
rior portions, either above the trabeculae or above the fissure 
(so-called pituitary fossa), and farther posteriorly to lie in the 
fissure itself; the recti externi extending still farther posteri- 
orly so that their hind ends lie under the notochord and hence 
beneath the basis cranii. That part of the space above the 
trabeculae, or above the fissure, thus occupied by the rectus mus- 
cles is said to form a part of the cranial cavity, but to be closed 
toward the brain by a membrane which, in these embryos, ex- 
tends posteriorly to the tip of the notochord. Anteriorly, the 
edges of this transverse membrane are said to be attached to 
the side walls of the cranium, above its floor, the membrane 
thus there separating the cranial cavity into dorsal and ventral 
parts. Posteriorly, the membrane is said to stretch from one 
trabecula to the other, there closing the intertrabecular fissure, 
(the so-called pituitary fossa) and taking part in the formation 
of the basis crani. It is, however, further said that, in later 
stages (embryos 18-mm. long), this same posterior portion of 
the membrane chondrifies, and that the cartilage so formed 
extends posteriorly to the tip of the notochord and there forms 
both the roof of the eye-muscle canal (myodome) and the floor 
of the cranial cavity. This cartilage is thus evidently the pro- 
otic bridge, and as the bridge cannot possibly have been formed 
by the chondrification of a membrane extending from one tra- 
becula to the other, there is some error in the descriptions. 
The myodome, as above described, is said by Schleip to pre- 
sent three sections: an anterior one, intracranial in position, 
but separated from the brain by the transverse membrane above 
