ZOOLOGY: E. P. ALUS 
243 
pituitary veins, the other recti muscles retaining their insertions on the exter- 
nal surface of the preclinoid wall, and so gave rise to the conditions found in 
Amia. The conditions found in the non-siluroid Teleostei then arose as a 
result of the resorption of the cartilage that, in Amia, forms the preclinoid 
wall, the pedicel of the alisphenoid, and those ventral portions of the b si- 
capsular commissures that form the lateral walls of the subpituitary portion 
of the myodome. Because of the resorption of the preclinoid wall, and its re- 
placement by membrane, the musculi recti interni, which in Amia have their 
points of insertion on either lateral edge of that wall, have first sought firmer 
attachment on the dorsal surface of the parasphenoid, and have then later 
pushed posteriorly in the open ends of the persisting portions of the canales 
parabasales. The fusion of these two canals with each other then formed a 
ventral myodomic compartment, which is, in early embryos, separated from the 
dorsal and primary compartment by membrane only; but this membrane 
may undergo either partial chondrification (Hyodon), or ossification (Gas- 
terosteus) the bone, in the latter case, forming a transverse and inclined 
ridge on the dorsal surface of the parasphenoid. The membranes result- 
ing 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 that is 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 con- 
nection 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 ascending process of the parasphenoid, and the tissues resulting 
from the resorption of the pedicel of the alisphenoid ossified, in certain fishes 
(Cottus, Gasterosteus), to form an anterior portion of that same process. 
In the Selachii the myodomic cavities of the Holostei and Teleostei are rep- 
resented either by canals in the basis cranii that are traversed by the pituitary 
veins and the internal carotid and efferent pseudobranchial arteries, or by a 
posterior and deeper portion of the large pituitary fossa of the chondrocranium 
that is shut off from the cavum cerebrale cranii by the dura mater, and is 
traversed by the pituitary veins and the internal carotid arteries. 
In embryos of Ceratodus there is a subpituitary space, traversed by the 
pituitary veins, that corresponds to the dorsal compartment of the teleostean- 
myodome, and the internal carotid canals of Amia have been added to it. 
This fusion of these canals with the dorsal myodomic cavity is due either to 
the resorption of the cartilage that separates them in Amia, or to a shifting 
posteriorly of both the hypophysis and the internal carotides from a position 
between the hind ends of the trabeculae to one between the so-called anterior 
prolongations of the parachordals. 
