LATERAL CANAL SYSTEM OF SELACHIANS 35 



to favor the conclusion that the thickened patches of the sen- 

 sory cords were originall}^ a part of the auditory thickening and 

 such was the interpretation of Mr. Swezey. 



It must be said, however, that my observations on the rela- 

 tion of the primordia to the auditory thickening are not of a 

 definite character and, as a result, they do not throw light on 

 the question of the independence of origin of the primordia of 

 the lateral line system and of the auditory epithelium (Landacre 

 '10, Reed '16). 



My own observations of developmental stages began with 

 embryos 9 mm. in length. In this stage definite rudiments of 

 the lateral sensory cords had appeared as well as of the principal 

 canals of the head. In a 36 mm. embryo the lateral sensory 

 cord of the trunk has reached the tail region and the distribu- 

 tion of the sensory cords of the head, although not in their 

 final arrangement, are so clearly outlined that the different lines 

 can be easily defined. 



Nine millimeter stage. The 9 mm. embryo, the head of 

 which is shown in surface view in figure 26, has about sixty 

 somites. The cranial ganglia from the fifth backwards are 

 well developed and can be seen in surface \dew as elevations of 

 more or less prominence. The ninth nerve, which may be 

 adopted as a landmark, can be identified by its ganglion just 

 behind the ear capsule and a long process extending from the 

 ganglion into the first branchial arch. 



Immediately posterior to the ninth are the lateralis and vagus 

 roots, the latter connecting with the walls of the medulla in a 

 broad sheet. The distal margin of this sheet exhibits, at this 

 stage, three ganglionic enlargements that lie above the second, 

 third and fourth branchial arches and have a placode connec- 

 tion with the thickened ectoderm. 



A dissection of this region shows that the ganglion of the 

 lateralis nerve is independent of the vagus. It is attached an- 

 terior to the vagus roots (figs. 27, 30) and crosses obliquely the 

 expanded sheet of the vagus rootlets (fig. 30). 



A cross section (fig. 28) taken through the plane of the line 

 A-B of figure 27 shows the structure of the lateralis ganglion 



