272 EDWARD PHELPS ALLIS, JR. 
the brain: a parachordal cartilage which extends from the 
transverse plane of the root of the nervus glossopharyngeus to 
that of the root of the nervus trigeminus; a polar (Pol) cartilage, 
which lies lateral to the anterior end of the notochord and ex- 
tends, in a direct anterior prolongation of the line of the para- 
chordal, from the root of the trigeminus to about the middle 
of the length of the hypophysis; and a trabecular cartilage, 
which, lying in the line prolonged of the other two cartilages, 
extends from about the middle of the length of the hypophysis 
to a point in front of the nervus opticus. The fundament of 
the musculus rectus externus of either side lies directly against 
the related polar cartilage. 
In embryos of this fish 11 to 12-mm. in length the adjoining 
ends of the parachordal polar, and trabecular cartilages of 
either side have fused with each other to form a continuous 
cartilage, the part formed by the polar and trabecular cartilages 
lying, as shown in the figures, slightly dorsal to the level of the | 
anterior end of the notochord. The trabeculae of opposite 
sides have fused with each other anterior to the recessus pre- 
opticus, thus enclosing a large fenestra basicranialis, into the hind 
end of which the anterior end of the notochord projects. The 
polar cartilages now occupy the positions of the so-called an- 
terior prolongations of the parachordals of Swinnerton’s and 
Gaupp’s descriptions of Gasterosteus and Salmo, and hence 
of the ‘Balkenplatten’ of Stohr’s descriptions of Salmo. The 
recti externi have now become inserted on the polar cartilages, 
and, because of this or for some other reason, the fenestra basi- 
cranialis is there slightly constricted. The fenestra encloses the 
ventral portions of the infundibulum and recessus preopticus, 
and in later stages the hypophysis and saccus vasculosus come 
to lie, respectively, in the interpolar and interparachordal por- 
tions of it. 
In embryos 14 to 20-mm. long the region under consideration 
has not changed in any important respect. The planum orbi- 
tonasale, formed by the fusion of the anterior ends of the tra- 
beculae, begins immediately anterior to the recessus preopticus 
and extends forward beyond that part of the lamina terminalis 
