4o8 



CHORD AT A. 



(VIJ, facial (VII.), auditory (VIII.), glossopharyngeal (IX.) 

 and vagus (X.). In Amniota two more are added — the 

 spinal accessory (XT.) 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 Cyclostomatd) 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. 



^ Optic 



f ' S Vesicle. 



Mid-brain 



^r 



Secondary Optic Vesicle. 

 Epiblast. \^ Optic Vesicle. 



Lens. / ^^V Optic Stalk. 



Lens. 



Epiblast. 



Choroid'Fissure. 



Primary Optic Vesicle. 



Pigment Layer of Retina. 

 Sensory Layer of Retina. 



Optic Stalk. 



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 Amniota the surface of the 

 olfactory sacs is kept perpetually moist by gland-cells, and 

 they acquire internal openings or internal 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 



