152 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
filled, like all meningeal spaces, with an albumen-containing cerebro- 
spinal fluid. 
From the urodeles upward there is an increasing division of the 
meninx primitiva into two layers, a pia mater bearing the blood- 
vessels and lying close to the cord, and a dura spinalis, separated from 
the pia by a subdural space, the perimeningeal space now being known 
as the peridural. In the mammals the pia becomes invaded by cavities 
separating a delicate arachnoid membrane from its outer surface, so 
that there is another space, the subarachnoid, in these forms. 
There may be slight differences in the region of the brain in the 
higher groups where the dura presses against and finally unites with the 
endorhachis, forming the dura mater of human anatomy, thus obliterat- 
ing the subdural space. In the mammals and to 
a less extent in birds the dura mater forms two 
strong folds. One of these is longitudinal and 
presses in between the two cerebral hemispheres as 
a firm membrane, the falx cerebri. The other 
fold, the tentorium, is transverse, and is inserted 
between cerebrum and cerebellum. It is occasion- 
ally ossified and united to the skull. 
THE BRAIN IN THE SEPARATE CLASSES. 
CYCLOSTOMES.—The brain is very different in the 
two classes of cyclostomes. All parts lie in the same hori- 
zontal plane, the flexures having disappeared, and the 
whole presents a primitive, almost embryonic appearance. 
In the lampreys the somewhat slender brain is elongate 
and its roof is largely epithelial, this extending to the mid- 
brain, of which only the hinder part is nervous in the middle 
line. The small cerebral hemispheres are largely com- 
High 5g; —eaia of posed of the corpora striata and the dorsal part of the 
Bdellostoma (Princeton, pallium is purely epithelial, the ventricles being well de- 
2204). 0, skeleton of veloped and extending into the olfactory lobes. The 
olfactory organ, the: _. i 
brain behind this; Vx, Ptic lobes and the medulla are relatively broad, but 
nerves. the cerebellum is reduced to an inconspicuous fold in front 
of the fossa rhomboidea. 
Authors do not agree regarding the interpretation of some parts of the myxinoid 
brain. The whole is much broader and shorter than in the other class and is 
marked dorsally by a groove running the whole length. According to Retzius, the 
*twixt-brain of Myzxine is invisible from above and the cerebellum is large, com- 
pletely covering the fossa rhomboidea. The cavities are greatly reduced, the 
