130 THE FROG. 



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

 themselves have undergone but little further change. At the 

 level of the notochord the external epiblast presents, on each 

 side, a very distinct and localised thickening of its inner or 

 nervous layer. This thickening projects inwards, and lies very 

 close to the pneumogastric nerve, a little below the middle of 

 its length, but as yet the two structures are independent. 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 epiblastic thickening and 

 the nerve have fused, and together form the ganglion of the 

 pneumogastric : 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 appear- 

 ance it is clearly a ridge-like thickening of the inner surface 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. Besides the main lateral line nerve described above, 

 other similar cutaneous branches are formed in connection with 

 the pneumogastric ganglion ; a more slender nerve is developed 

 nearer the mid-dorsal line ; and 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 



