DEVELOPMENT OF PETROMYZON FLUVIATILIS. 359 



is formed, which arises at about the level of the notochord 

 (fig. 32). This heap of cells arises close behind the lens of the 

 eye, but seems to be distinct from it. It is divided into a 

 larger anterior part, which belongs to the fifth nerve, and a 

 smaller posterior portion, which forms the ganglion of the 

 seventh. The roots of the nerves seem to me — though it is 

 difiicult to be certain on this point — to arise as outgrowths 

 from a neural ridge in the lateral surface of the brain ; these 

 grow down and fuse with epiblastic thickening. This origin 

 of the roots of the nerves corresponds with that described by 

 Balfour, Marshall, Van Wijhe, and Beard, in the 

 Elasmobranchs, and differs from what occurs in the Am- 

 phibia as described by Spencer, where the nerve also is 

 derived from the inner layer of epiblast. As Spencer sug- 

 gests, this is probably due to the presence of a double layer of 

 epiblast, the epidermic and nervous, in the Amphibia. 



By the nineteenth day the ganglion of the fifth nerve has 

 completely separated off from the skin. It has now divided 

 into two portions, which have, however, a common root taking 

 its origin from the hind-brain just in front of the ear. The 

 most anterior part forms a large ganglion on the root of a 

 nerve which runs over the eye (fig. 22). This is the oph- 

 thalmic ganglion, and the nerve is the ophthalmic branch of 

 the trigeminus; it probably corresponds with the portio- 

 profundaof the ophthalmicus superficialis of the Elasmobranchs. 

 Immediately behind the ophthalmic ganglion, but quite dis- 

 tinct from it, lies the ganglion of the other half of the fifth 

 nerve. From this a mandibular nerve proceeds to run close 

 behind the mouth, and later a maxillary branch appears prse- 

 orally. In the angle between these ganglia the eye lies. The 

 nerve connecting the ophthalmic with the main ganglion of 

 the fifth nerve, described by Ahlborn in the adult, is not found 

 at this stage, and both the ganglia are of approximately equal 

 size. 



The seventh nerve arises behind the fifth and enters its 

 ganglion, which, when separated off from the epiblast, lies close 

 in front of the ear capsule (fig. 38). In early stages, 



