436 



L. S. STONE 



Stage 20-27 



Neural crest. In the description of the following stages, the 

 descending bands of neural crest will be designated by the vis- 

 ceral arches upon which they lie. 



Fig. 16 Frontal section, showing in stage 25 crest cells over dorsal portion of 

 right optic vesicle and over mesoderm of hyoid and branchial arches. Also shows 

 positions of VII lateral-line placode and epibranchial placodes of IX and X. 

 X 37. 



li Fig. 17 Frontal section, showing crest cells migrating over mesoderm of 

 hyoid and branchial arches and above optic vesicles in stage 26-27. Also shows 

 positions and relative thickness of VII and X lateral-line placodes and epibran- 

 chial placodes of IX and X. X 37. 



Fig. 18 Frontal section, showing ophthalmic placode on right side lying 

 close to anterior portion of neural crest migrating upon mesoderm of mandibular 

 arch in stage 26-27. X 37. 



Further rapid growth of neural crest has taken place ventrally, 

 accompanied by a thinning out of the crest cells over the niid- 

 dorsal portion of the brain (fig. 4). The anterior border of the 

 eye is traversed as far as the dorsal border of the nasal placode 

 by a narrow strip of loosely arranged cells. The mesoderm of the 



