438 L. S. STONE 



epibranchial placodes which contribute placodal cells to form the 

 visceral sensory ganglion of X. The placode of the vagus lateral- 

 line ganglion and of the midbody line is situated posterior to its 

 former position and lies across the anterior border of the first 

 somite above the neural crest of the second, third, and fourth 

 branchial arches (fig. 17). In the ectodermal intersegment of the 

 first and second somites is a prominent thickening, which later 

 is the position of the placode which gives rise to the dorsal 

 body line of sense organs and a portion of the vagus lateral- 

 line ganglion. 



Stage 30 



Neural crest. This stage shows the rapid descent the neural 

 crest has made in its progression ventrally. No longer are the 

 crest cells from both sides continuous over the dorsal border of 

 the brain (fig. 5) . The crest cells over the mesoderm of the man- 

 dibular arch and over the dorsal and anterior borders of the eye 

 have broken all connection with those of the hyoid arch. The 

 maxillary and mandibular processes of the mandibular arch have 

 become covered with crest cells. The manner in which the meso- 

 derm of the visceral arches is becoming completely surrounded 

 by a layer of crest cells two to three cells thick is shown in figure 

 19. The mesoderm of the mandibular and hyoid arches is com- 

 pletely surrounded by the crest cells, while the first branchial arch 

 is enclosed around its anterolateral and posterior borders. The 

 crest cells over the anterior border of the eye now extend as a 

 narrow band to the dorsal border of the nasal placode. A few 

 scattered crest cells are found along the dorsal border of the 

 auditory vesicle which are continuous with crest cells lying on 

 its anterior and posterior borders. A few crest cells come to 

 lie below the ventral border of the auditory vesicle, due to the 

 rapid ventral shifting of crest cells from above. The crest cells 

 from the anterior border of the auditory vesicle continue with 

 those of the hyoid arch, which have descended ventrally farther 

 than the crest cells of the first branchial group. The dorsal 

 region of the crest cells lying posterior to the auditory vesicle 

 consists of loosely arranged cells which extend to the middle of 



