130 THE FEOG. 



In embryos of about 4 mm. length (c/. Fig. 61), the nerves 

 themselves have undergone but little further change. , At th e 

 le vel of the notochord th e externa l e piblast presents, on ea ch 

 s ide, a v ery distinct and localised thickening of its inner or 

 nervous layer . T his thickening projects inwards, and lies v ery 

 c lose to the pneumogastric nerve, a li ttle below the midd le of 

 its length, but as yet the two structures are independ ent. The 

 thickening is well marked, and extends horizontally backwards 

 a little distance beyond the nerve. 



At the time of hatching, i.e. in tadpoles of about 7 mm. 

 length (cf. Figs. 69, 72, and 73), the e piblastic thickening and 

 the nerve have fuRe(^ . and together for m the ganglion of t he 

 pnenmofrastric : the horizontal extension backwards of the 

 thickening, which forms the lateral line nerve, has grown enor- 

 mously, reaching now almost to the hinder end of the body of the 

 tadpole. The mode in which this lateral line nerve grows has not 

 been determined with certainty in the frog ; at its first appe ar- 

 a nce it is clearly a ridge-like thickening of the inner surfac e of 

 the epiblast. but it is difficult to decide whether the extension 

 backwards, which is effected with great rapidity, is due to a 

 splitting off from the epiblast, or to growth backwards of a solid 

 rod of cells from the ganglion of the pneumogastric. Such 

 evidence as is forthcoming rather favours the latter view. In 

 transverse sections at this stage (Fig. 82, nl), the lateral line 

 nerve has the appearance of a solid rod of cells, lying in a groove 

 along the inner surface of the epiblast, at the level of the lower 

 part of the spinal cord. The lateral line nerve is of large size 

 throughout the whole period of tadpole life ; it is present during 

 the metamorphosis, but disappears completely at its close. 

 During the later tadpole stages it separates from the skin, and 

 becomes more deeply placed among the muscles of the body 

 wall. TtpsiHftHjjip, Tna,in, 1atera,1 line nprvn dftgp.p b_ed ab ove, 

 ot her similar cutaneous branches are formed in conn ection with 

 tfae pneum ogastric ganglion ; a more slende r nerve is develo ped 

 ne arer the mid-dorsal line ; ands a stout nerve runs at first 

 ventralwards from the ganglion, and then backwards along the 

 sides of the ventral surface of the abdominal region. 



Concerning the further development of the pneumogastric 

 nerve itself there are some points of interest. The root of 

 attachment to the brain, which is acquired in the same manner 



