314 EDWARD PHELPS ALLIS, JR. 
the dorsal aorta has been excluded from it because of the for- 
mation of a circulus cephalicus. What the primary relations 
of the hypophysis and pituitary veins to this preexisting canal 
were is problematical, but they became lodged in its anterior 
portion and so gave rise to the conditions actually found in 
Lepidosteus and Polypterus. The musculi recti externi then 
secondarily invaded this space by traversing the foramina for 
the pituitary veins, the other rectus muscles retaining their in- 
sertions on the external surface of the preclinoid wall, and so 
gave rise to the conditions found in Amia. The conditions in 
the non-siluroid Teleostei then arose as a result of the resorption 
of the cartilage which, in Amia, forms the preclinoid wall, the 
pedicel of the alisphenoid, and those ventral portions of the 
basicapsular commissures which form the lateral walls of the 
subpituitary portion of the myodome. Because of the resorption 
of the preclinoid wall, and its replacement by membrane, the 
musculi recti interni, which in Amia have their points of in- 
sertion on either lateral edge of that wall, have first sought firmer 
attachment on the dorsal surface of the parasphenoid, and have 
later pushed posteriorly in the open ends of the persisting por- 
tions of the canales parabasales. The fusion of these two canals 
with each other has formed a ventral myodomic compartment 
which, in early embryos, is separated from the dorsal and pri- 
mary compartment by membrane only; but this membrane may 
undergo either partial chondrification (Hyodon) or ossification 
(Gasterosteus), the bone, in the latter case, forming a transverse 
and inclined ridge on the dorsal surface of the parasphenoid. 
The membranes resulting from the resorption of the preclinoid 
wall were then pressed together in the median line by the recti 
interni, and form a median vertical myodomic membrane which 
encloses the internal carotid arteries in a membranous canal, 
the homologue of the cartilaginous canals of Amia. The efferent 
pseudobranchial arteries, pressed downward by the recti interni, 
lost their connections with the internal carotids and acquired a 
cross-commissural connection with each other. The membrane 
resulting from the resorption of the anterior portions of the 
basicapsular commissures of either side ossified as part of the 
