408 CHORDATA. 
(VI.), facial (VII.), auditory (VIII.), glossopharyngeal (IX.) 
and vagus (X.). In Amniota two more are added — the 
spinal accessory (XI.) and hypoglossal (XII.). 
From the spinal cord there arises a series of spinal nerves, 
each of which has a dorsal (sensory) and ventral (motor) 
root, the two uniting soon after emergence from the spinal 
cord. 
Sense-Organs.—The first sense-organs or olfactory 
organs arise as a pair (single in Cyclostomata) of epiblastic 
pits at the anterior end of the head. They form the 
olfactory sacs with a sensory epithelium. The fore-brain in 
development grows out in a pair of olfactory lobes which 
Fig. 295.—THREE STAGES IN THE DEVELOPMENT OF THE 
VERTEBRATE EYE, 
Fore-brain. Secondary Optic Vesicle. 
: Epiblast. : Optic Vesicle. 
_ Optic. Lens. Optic Stalk. 
Vesicle. 
Mid-brain. 
Primary Optic Vesicle. R 
Pigment Layer of Retina. 
Sensory Layer of Retina. 
Lens. @ Optic Stalk. 
Epiblast. 
ChoroidjFissure. Secondary Optic Vesicle. 
come into intimate contact with the sensory epithelium by 
means of the olfactory nerves. The lobes may be of great 
length, as in the skate. In the Amaio¢a the surface of the 
olfactory sacs is kept perpetually moist by gland-cells, and 
they acquire internal openings or zuéernal nares into the 
buccal cavity. They then form a passage for the current of 
respiratory air. 
The second sense-organs or eyes arise from three sources. 
The fore-brain grows out laterally into two primary optic 
vesicles towards the skin. These take the form of a round 
