THE NERVOUS SYSTEM OF AMPHIBIA 279 



D. THE EARLY SWIMMING STAGE 



Of the general cutaneous system the lateral and mesial ter- 

 minal branches of the r. ophthalmicus profundus, the r. maxil- 

 laris, r. mandibularis with a nerve to the balancer, and a nerve 

 from the ophthalmic ganglion directly to the overlying skin are 

 well defined (fig. 4). The r. auricularis vagi is perceptible but 

 its distribution is uncertain. The root fibers of the trigeminus 

 now extend in the descending tract to about the level of the 

 root of the vagus nerve, and this tract is joined by the root of 

 the vagus ganglion (fig. 6). The giant ganglion cell component 

 of the trigeminal nerve is still present, but its exact distribution 

 through the nerve can not be determined. The hyomandibular 

 lateral line primordium is now continuous with the other pre- 

 auditory primordia and is innervated by the r. hyomandibularis 

 VII. The other lateral line primordia are now innervated prac- 

 tically throughout their entire extent, the lateral Une of the 

 trunk extending into the level of the seventeenth myotome. 

 The extent of the ascending and descending tracts of the lateral 

 Hne roots is shown in figure 0, where the extent of the descending 

 auditory fibers is also figured. The visceral sensory component 

 is now represented by a well developed r. alveolaris VII, r. lin- 

 gualis IX, and a less developed first r. branchiahs vagi. There 

 are no prebranchial or pharyngeal branches of the postauditory 

 branchial nerves. The vagus root now connects with the brain 

 and contributes fibers to the fasciculus solitarius. In the retina 

 mitosis is still going on rapidh' in the middle of the external 

 layer, while in the internal layer ganglion cells are already 

 developed which send their axones along the optic stalk into the 

 brain. They decussate in the most rostral portion of the post- 

 optic commissure. Only a suggestion appears of neuroblasts in 

 the region of the bipolar cell la^'er. The olfactory nerve is well 

 established and its fibers distribute themselves in a very definite 

 glomerular zone of the olfactory lobe. 



