448 L. S. STONE 



Stage 35 + 



Neural crest. With the exception of a more compact arrange- 

 ment in the branchial and hyoid regions, there is but Uttle change 

 in the neural crest over the mesoderm of the branchial and hyoid 

 arches (fig. 8). No crest cells are found over the dorsal and 

 anterior borders of the auditory vesicle. Above the branchial 

 arches and posterior to the auditory vesicle loose crest cells lie 

 in the same relation to the branchial bands as in stage 35. In the 

 mandibular region there has been but little change in the relation 

 of the crest cells except for a steady decrease in the number of 

 cells dorsal to the eye. Beyond this stage the neural-crest ar- 

 rangement is obscured by the lack of color differentiation, by the 

 further migration of crest cells to the median sides of the meso- 

 derm of the arches and by the rapid growth of the external gills, 

 so that further knowledge of crest-cell arrangement cannot be 

 gained by dissection. The manner in which the crest cells as- 

 sume their positions around the arches may be more clearly shown 

 in the description of the process which takes place in stage 36. 



Placodes. The primordia of the supra- and infra-orbital 

 lateral-line sense organs, from their point of fusion posterior to 

 the eye, curve over the dorsal and posterior borders of the eye, 

 respectively, as narrow, consolidated, rod-shaped thickenings in 

 the ectoderm (fig. 8). The supra-orbital primordium, when 

 compared with its appearance in stage 35 (fig. 7), shows a more 

 decided change in its shape than does the infra-orbital primor- 

 dium. Posterior to their point of juncture, the primordium of the 

 hyomandibular lateral-line sense organs extends ventrally along 

 the upper portion of the hyomandibular cleft as a short rod similar 

 in character to that of the supra- and infra-orbital primordia. 

 The primordium of the ventral hyomandibular group of lateral- 

 line sense organs now extends farther ventrally from the posterior 

 border of the base of the balancer in the lower portion of the 

 hyomandibular cleft and its rod-shaped placode has somewhat 

 increased in depth. In addition to the primordia shown in the 

 previous stage (fig. 7) , there is now a primordium of the mandib- 

 ular group of lateral-line sense organs extending as a short 

 ectodermal thickening along the ventral border of the base of 



