RATIIKE ON THE DEVELOPMENT OF THE CRANIUM. 239 
Setting aside a few Cyclostomes, the mass in question accumulates 
more especial!) 7- on the two sides of the cephalic part of the notochord ; 
so that the sheath which it forms acquires, as it were, two wings, 
which likewise are situated within the other and less coarsely 
granular substance of the base of the capsule, which encloses the brain 
and its membranes—in other words, they are situated in the future 
basis cranii. They attain a different relative breadth in different verte- 
brata, but in general are larger and broader than the third cerebral 
vesicle, which lies over them and the cephalic part of the notochord. 
Hence their origin appears to be connected with the circumstance 
that the capsule, formed by a part of the serous layer of the germ, 
which contains the brain, is necessitated to acquire a much greater 
breadth than the capsule surrounding the spinal marrow. 
Furthermore, the investing mass of the cephalic part of the 
notochord is continued forwards for a certain distance beyond this 
part—the sheath which it forms, and its two wings, passing into a 
horizontal plate, which also forms a part of the basis cranii, and 
reaches almost close to the posterior end of the infundibulum. Here, 
however, it divides, or rather sends out certain processes, the number 
of which is different in different animals. In Fishes and Batrachia 
only two such processes occur, and these are situated in the lateral 
halves of the skull, and have a similar form and direction. They lie 
enclosed in the general gelatinous basis cranii, extend even origi¬ 
nally, in all probability, not merely as far as the anterior end of the 
latter, but also a certain distance beyond it. Posteriorly, they lie at 
a moderate distance from one another, but at the anterior end of 
the brain case (while this has still a generally gelatinous consistency), 
they come close together, and, in front of it, bend outwards like 
horns. Two similar processes occur also in all the higher animals. 
But, in addition to these, in the earliest period of the foetal life of 
these last, a third, azygos process is present, which passes off be¬ 
tween the two first from the anterior edge of the tabular projecting 
part of the investing mass. It is shorter than the two others and 
projects into the cranial cavity, being bent upwards and forwards. 
Its convex side is directed forwards, its concave side backwards, and 
it lies between the first and third cerebral vesicle in the deep sinus, 
which the brain of all animals above the Batrachia forms—by 
bending upon itself — and by its end it touches the lower wall of 
the middle cerebral vesicle. The brain appears in these higher 
animals to have been bent round the azygos process, while in Fishes 
and Batrachia it always retains a tolerably straight direction. 
These processes, which the investing mass of the notochord sends 
out, may be termed the rafters of the skull (trabeculae cranii). 
§. 13. a. In the Snake, the Blindworm, the Lizard, the Fowl, the Pig, 
and the Buminants, the pointed cephalic part of the notochord extends 
only to between the auditory capsules. It is surrounded by a dense 
blastema, which projects beyond it more or less laterally, but, ante¬ 
riorly, extends only as far as the infundibulum, and forms a more or 
