46 THE ORIGIN OF VERTEBRATES 
by the growth of the nerve-masses on each side of it. Throughout 
the whole vertebrate kingdom this obliterated portion still leaves 
n its mark as the raphé or seam, 
which is so characteristic of 
the infra-infundibular portion 
of the brain. 
Here, again, it is seen how 
simple is the explanation of a 
peculiarity which has always 
puzzled anatomists —why 
should there be this seam in 
the infra-infundibular portion 
of the brain and not in the 
supra-infundibular or in the 
spinal cord? The correspond- 
ing compression in the upper 
brain-region forms the lateral 
ventricles, as is seen in the 
accompanying figure of the 
brain of Ammoccetes (Fig. 22), 
In yet another instance it is 
seen how markedly the nervous 
masses are arranged in the 
Cr., membranous cranium; J, olfactory pale poston with PEE pEEN ta 
nerves; l.v., lateral ventricles; gl., glan- the central tube as are the 
dular tissue which fills up the cranial perve ganglia with respect to 
li the intestinal tube in the case 
of the invertebrate. Thus in birds a portion of the spinal cord 
in the lumbo-sacral region presents a very different appearance 
from the rest of the cord; it is 
known as the rhomboidal sinus, 
and a section of the cord of an 
adult pigeon across this region is 
‘iy, given in Fig. 23. As is seen, the 
nervous portions are entirely con- 
Fic. 23.—SEcTION THROUGH RHOMBOIDAL fined to two masses connected 
Sinus, Oe PIED, together by the white anterior 
commissures which are situated laterally and ventrally to a 
median gelatinous mass; the small central canal is visible and 
A 
Fic. 22,HorizonraL SECTION THROUGH 
THE BRAIN OF AMMOCGTES. 
