The Development of the Cartilaginous Skull ete. in Neeturus. 415 
still contain large yolk granules, while their nuclei are separated 
from one another much farther than are those in the procartilage of 
the branchial arches. In both tissues there is, however, a protoplasmic 
differentiation, which is peculiar to procartilage. It readily stains, 
but does not resemble the clear matrix of true cartilage. 
The basicranial plate, above described, extends from the first 
spinal nerve, the anterior of the two hypoglossus nerves, to the 
infundibulum, becoming gradually broader, but varying in depth. It 
lies a short distance below the floor of the brain, and is prolonged, 
on either side of the infundibulum, in an anterior horn which extends 
forwards and downwards. The chorda runs through the long axis 
of the plate, but does not reach its anterior limit by several sections. 
The depth of the plate, except near the anterior extremity, is less 
than the diameter of the chorda whose sides it bounds. The chorda 
thus divides the plate into two halves united with one another 
anteriorly. The two anterior horns of the plate form the posterior 
part of the trabecular bars. The procartilage of the anterior part 
of the trabeculae is mesectodermic, resembling that of the branchial 
arches, yet the transition from the anterior mesectodermic procartilage 
of the trabecular bar to the mesodermie procartilage of the anterior 
horns of the basieranial plate is not sharply marked. As indicated 
by the distribution of yolk granules the point of union between the 
two kinds of tissue lies just posterior to the optic stalk. 
Fig. 10, pl. XVII, is drawn from a sagittal section through the head 
of an embryo of 14 mm, in which the yolk granules have not yet 
disappeared from the mesoderm. The section passes to one side of 
the long axis of the embryo, and through the region where the 
anterior horn of the basicranial plate unites with the mesectodermic 
part of the trabecular bar. The nuclei are represented in the figure 
by circles and the yolk granules by dots. Anterior to the posterior 
part of this bar of procartilage (/r), the lateral wall of the infundi- 
bulum is seen. Anterior to the ventral, or anterior, part of the bar 
lies the optic stalk. In the axial plane, the convex wall of the 
infundibulum meets the basicranial plate, which lies above the hypo- 
physis, as seen in fig. 7 4, pl. XVIII, a transverse section through the 
anterior extremity of the basicranial plate in an embryo of 15 mm. 
Although the basicranial plate and its anterior horns are meso- 
dermie in origin, while the cells that compose the anterior part of 
the trabeculae are of eetodermie origin, when true cartilage appears 
and the yolk granules are no longer found, I am unable to distinguish 
