■*. 



444 L. S. STONE 



to the hyobranchial cleft. Above the epibranchial placode of IX 

 there appears a small round thickening, possibly the IX lateral- 

 line placode, in the ectoderm near the posteroventral border 

 of the ear. It merges into thickened ectoderm which poste- 

 riori}^ is identified as the epibranchial placodal region of X (fig. 

 29). Above the first branchial cleft a placode is also giving off 

 cells in large numbers which lie close to the neural crest. It 

 is connected by loose cells which lie along the ventral border 

 of the vagus lateral-line ganglion next to a thickened portion 

 of the ectoderm (fig. 6). This entire mass of cells, including 

 the placode in the dorsal portion of the first branchial cleft, 

 is the placodal mass which gives rise to the visceral sensory 

 ganglion of X. The anterior extremity of the elongated plac- 

 ode over the posterodorsal border of the crest cells of the bran- 

 chial region is splitting off a large mass of placodal cells to 

 form the vagus lateral-line ganglion (fig. 28). The placode in 

 the intersegment of the first and second somites has increased 

 somewhat in size. 



Stage 35 



Neural crest. The remains of neural crest over the dorsal 

 portion of the eye exist only as a very narrow band of loose cells 

 which are continuous anteriorly as far as the nasal placode, and 

 posteriorly as far as the mandibular crest cells with which they 

 are united (fig. 7). The posterior border of the nasal placode is 

 covered by a group of crest cells continuous with the maxillary 

 group, which in turn joins ventrally in front of the stomadaeum 

 with the maxillary group of the opposite side. Only a few loose 

 crest cells are still visible along the anterior border of the auditory 

 vesicle. The crest cells extend to the ventral extremity of the 

 mesoderm of the hyoid arch near the pericardium. Above the 

 gill region and posterior to the auditory vesicle, a narrow longi- 

 tudinal band of crest cells extends to near the posterior border of 

 the first somite. Descending ventrally are bands of crest cells 

 which supply each of the branchial arches. The recent splitting 

 of the broad most posterior band of crest cells, which existed 

 in stage 30 (fig. 6), is indicated by the ventral continuations of 

 the crest-cell bands of the third and fourth branchial arches. 



