MYODOME AND TRIGEMINO-FACIALIS CHAMBER 309 
the intrapolar processes, and hence also the polar cartilages, 
must be hypochordal and not parachordal structures. This, 
then, is in accord with, but an extension of, Terry’s conclusion 
(17, pp. 344 and 396) that there seems no doubt of the presence 
of a cartilaginous hypochordal layer in the occipital region of 
mammals generally, and that the basal plate of the occipital 
region falls into the category of arch structures, not centra. 
The notochord and trabeculae may now be considered. Swin- 
nerton says that the notochord in Gasterosteus undergoes no 
actual reduction from the earliest to the latest stages examined 
by him, its relatively less extensive anterior prolongation in older 
embryos being wholly due to an anterior prolongation of the 
parachordal cartilages. Froriep, however, says (’02 a, ’02 b) 
that in Torpedo there is an actual disintegration of the anterior 
portion of the notochord. According to him, in early embryos 
of that fish, the notochord is separated into two definite regions, 
one of which he considers to be spinal and the other prespinal. 
The spinal region is said to begin at the dorsorostral corner of 
the first visceral pouch, this point coinciding with that in which 
the dorsal wall of the foregut, in early embryos, bends abruptly 
ventrally in an obtuse angle, the notochord there also bending 
ventrally at the same angle. Posterior to this point the noto- 
chord develops a cuticular -sheath immediately after its con- 
striction from the dorsal wall of the foregut, and is persistent 
throughout the life of the individual. Anterior to this point, 
and hence in the prespinal region, the notochord presents two 
different conditions, one related to the region in which the man- 
dibular head cavities develop and the other to that in which 
the premandibular cavities develop. In the mandibular region 
a chorda entoblast is said to be constricted from the dorsal 
wall of the foregut exactly as in the spinal region, but it does 
not undergo further differentiation and later entirely disinte- 
grates. In the premandibular region, according to Froriep 
(792\b, p. 55): 
Kommt es nicht einmal zur Bildung einer primitiven Chordaan- 
lage, sondern deren Bildungsmaterial sowohl wie dasjenige des Meso- 
blasts bleibt ungesondert in der Wand des Vorderdarms enthalten. 
