246 ALtOB JOHNSON AND LILIAN SHELDON. 



The 3rd nerve is seen at a later stage in fig. 20. Its 

 point of attachment has been shifted down the side of the 

 brain, and the nerve is directed forwards towards the eye. 

 We have not ascertained whether or no there is any sensory 

 thickening of the epiblast corresponding to it, but it seems 

 possible that the ciliary ganglion may be fused with the 

 Gasserian, as is stated by Mr. Beard (4) to be the case in the 

 Frog. It would thus not have a separate sense organ of 

 its own. 



The Trigeminal nerve grows downwards from the brain 

 till it reaches the level of the sensory epiblastic thickening, 

 and then fuses with it (fig. 21). The point of fusion con- 

 stitutes the Gasserian ganglion together with the sensory 

 thickening. It is not possible to decide if the epiblast actually 

 takes part in the formation of the ganglion. The mere 

 presence of dividing nuclei in this region, as insisted on by 

 Mr. Beard, seems to us to prove nothing, since all the tissues 

 of the body are actively growing, and consequently contain 

 numbers of such nuclei. We are inclined, therefore, to think 

 that the fusion of the nerve with the epiblast is merely a case 

 of innervation of a sense organ, exactly comparable to what 

 occurs in the nose and ear, and that, in all such cases, the 

 nerve-elements are derived from the brain and the sense 

 elements from the epiblast. Professor Marshall has shown 

 how early this fusion occurs in the case of the ear in the 

 Chick (16). 



The root of the 5th nerve is at first attached to the dorsal 

 surface of the brain (fig. 18). Later, the surface of attach- 

 ment widens out and extends further down the side (fig. 32), 

 and then gradually becomes confined to a small area situated 

 about half way down the brain (fig. 33). The point of 

 attachment is thus shifted downwards, no secondary attach- 

 ment being formed in this case while the first is lost, as has 

 been described by Professor Marshall in the Chick (16) and in 

 Scyllium (17). 



The Gasserian ganglion is for a short time fused into one 

 mass with the sensory epiblast. Soon it begins to sink deeper 



