THE CRANIAL NEUVKS. 267 



become continuous with the olfactory epithelium. The mode in 

 which this connection is acquired is closely similar to that in 

 which the typical cranial nerve acquires connection with the 

 sensory patch of the surface epiblast, and it has been suggested, 

 with much reason, that the olfactory epithelium may be homo- 

 logous with one of these sensory patches. 



The condition of the olfactory nerve at the end of the fifth 

 day is shown in Fig. 115, i. The nerve, which is still very short, 

 runs downwards and backwards from the under surface of the 

 hemisphere to the olfactory pit. 



On the seventh day, as already noticed, the beak begins to 

 form ; and during this and the following days it grows forwards 

 with great rapidity. The olfactory sacs become imbedded in the 

 sides of the beak (Fig. 131, OK), and are carried forwards with 

 the beak as it lengthens. This causes a change in the direction 

 and in the relations of the olfactory nerves, which, previously 

 quiescent and inactive, have now to elongate rapidly, in order 

 to maintain the connection between the olfactory organs and the 

 brain. This elongation is effected mainly by growth of the 

 nerves themselves, but partly, as already explained, by pulling 

 out of the anterior ends of the hemispheres, from which the 

 olfactory nerves arise, to form the olfactory lobes (Fig. 116, BY). 



It is very possible, therefore, though not yet proved, that 

 the olfactory nerve is really comparable to a typical cranial 

 nerve, such as the facial, in which the sensory cutaneous branch 

 is the only one developed. 



II. The optic, or second cranial nerve. The optic nerves in 

 the chick are very generally described as being formed directly 

 from the constricted necks, or stalks, of the optic vesicles, which 

 connect these with the brain. If this be correct, the optic nerve 

 is in no way comparable with the other nerves, cranial or spinal, 

 but must be contrasted with all of these as being formed by 

 direct modification of part of the brain walls. 



There are, however, strong grounds for suspecting that, as 

 in the frog (p. 139), the fibres of the optic nerve really arise in 

 the retina, and grow inwards to the brain; the optic stalk afford- 

 ing the path along which they grow, but not itself taking any 

 direct part in their formation. 



The neural ridges, as already described, extend forwards 



