RATHKE ON THE DEVELOPMENT OF THE CRANIUM. 241 
they approach one another more and more, and thrust the anterior 
part of the brain upwards, so as to remove it further and further 
from the trabeculae, which have already coalesced for a greater or less 
distance. In consequence of this operation, the coalesced trabeculae 
send upwards a perpendicular plate, which forms a septum between 
the eyes, and either consists entirely of cartilage, or, to a certain 
extent, also becomes membranous. Among the animals the develop¬ 
ment of whose skulls I have examined, this plate exhibits its pro¬ 
portionally greatest height in the Chick, the smallest in the Blind- 
worm. # In the Snakes and Mammalia, however, it is not even 
indicated; on the contrary, in them the brain remains lying directly 
upon the trabeculae of the skull. 
c. The ossification of the basis cranii takes place in the Snake, 
the Blind-worm, and the Lizard, on the whole, in the same way. 
The basi-occipital is formed by the ossification of the previously 
chondrified investing mass of the cephalic part of the notochord, as 
Well immediately around this part, as in its two wings or lateral 
expansions, by which a bony plate is produced, which anteriorly 
hardly extends perceptibly beyond the notochord, and at first 
encloses it, as its axis. Gradually, however, the cephalic part of 
the notochord disappears, and the space which it occupied becomes 
filled by a growth of the bony substance in question. Hence, in 
these animals, the basi-occipital is formed in just the same way as 
the body of a vertebra, and the differences between them relate 
mainly to external form, the basi-occipital being much broader, in 
relation to its thickness, than an ordinary body of a vertebra; it is, 
in fact, a plate and not a cylinder. 
A second bony plate appears at a considerable distance in front 
of the first; namely, in that half of the investing mass which projects 
beyond the cephalic part of the notochord, close behind the three 
trabeculae, which pass forward as processes of this mass. This second 
bony plate is the basi-sphenoid. At first, as has been said, it lies at a 
considerable distance from the first plate; gradually, however, it 
approaches the latter more and more, whilst, on the one hand, new 
bony matter extends from it posteriorly into the investing mass, and 
on the other, the basi-occipital grows in the same manner towards it. 
In time the two bony plates come into contact and coalesce. The 
basi-sphenoid, therefore, though it is still in the investing mass of the 
notochord, does not arise in quite the same manner as the body of 
an ordinary vertebra, since it never incloses a part of the notochord, 
but rather is formed in front of the notochord, and from the very 
beginning represents a quite dense thick plate. Neither can it be 
united with the basi-occipital as the vertebral centra are united since 
a part of the sheath of the notochord is an essential part of such 
union (§4). 
* In many osseous fish, a similar, very large and high septum is formed. It is 
probably largest of all in the Sword-fish, whose eyes have such a prodigious size. 
N. H. E.—1863. E 
