LATERAL CANAL SYSTEM OF SELACHIANS 37 



and its connection with the brain wall. At this stage the gan- 

 glion cells are of the bipolar type and the section shows three 

 central processes penetrating the wall of the medulla. The 

 peripheral processes run obliquely backwards toward the cells 

 of the ectodermal anlage of the lateral sensory cord. 



Eleven millimeter stage. This stage (fig. 29) shows well 

 marked advances in the development of cranial structures. 

 Confining attention to the vagus and lateralis region, surface 

 observations show the widely expanded sheet of vagus rootlets 

 merging on the ventral margin into three well marked vagus 

 ganglia. The lateralis ganglion is connected with the brain 

 wall in front of this widely expanded sheet and the ganglion 

 itself passes obliquely backwards over the sheet of rootlets 

 and above the vagus ganglia. This is shown better in the dis- 

 section of this region of a slightly older embryo (fig. 30). The 

 independence of the lateralis ganglion is clearly shown in this 

 sketch and sections confirm this independence, although its 

 ganglion cells and those of the vagus are closely associated in 

 some of the sections. Distally the fibers arising from the gan- 

 glion cells of the lateralis pass out among the growing cells of 

 the ectodermal cord. The terminals of the peripheral fibers 

 end freely among the cells and have not as yet established 

 dendritic relations with cells of the ectodermal cord. The 

 condition is shown in a little later stage of development in 

 figures 35 and 36. 



In the 11 mm. stage the ectoderm at the growing end of the 

 lateral sensory anlage is crowded with di\'iding cells which lie 

 next to the surface (fig. 31) and the cells of this thickening are 

 enlarged or elongated. 



Thirteen millimeter stage. The sketch of this stage (fig. 32) 

 is copied from a drawing by Mr. Otto Swezey. Four gill-slits 

 have broken through and fringes of external gills are beginning 

 to appear. Owing to thickening of the adjacent tissues, the 

 cranial ganglia and their roots are not so clearly discernible 

 from the exterior. 



Two small thickened cords of ectoderm fork in front of the 

 facial ganglion. One is directed above and the other below the 



