434 L. S. STONE 



thickened portion of the ectoderm which is in close contact with 

 the crest cells at its postei^or border. The longitudinal ectoder- 

 mal ridge (fig. 2) is still prominent, but extends slightly more an- 

 teriorly and less posteriorly than in the previous stage. The 

 thickening in the dorsal portion of the hyomandibular cleft is 

 slightly larger, but still remains crescentic in shape (fig. 14). 

 The approximate positions of two placodes, anterior and posterior 

 to the probable position of the auditory placode, are indicated by 

 dotted lines. That which lies just anterior to the auditory 

 placode gives rise to part of the lateral-line ganglion of VII. 

 That which lies posterior to the auditory placode gives rise to 

 part of the lateral-line ganglion of X. The positions of other 

 placodes cannot be exactly located in sections of this stage by 

 the appearance of any ectodermal thickenings. 



Stage 25 



Neural crest. A further very rapid ventral growth of the 

 neural crest has taken place, the most pronounced proliferation 

 involving those cells which are in the region over the dorsal 

 border of the eye (fig. 3) , The upper portion of the mandibular 

 arch as far as the middle of the posterior border of the eye is 

 covered by the neural crest. A growth of the neural crest passes 

 over the dorsal border of the eye. A section passing through 

 the upper half of the optic vesicles (fig. 15) shows on one side 

 along the anterior border of the latter a transection of the ven- 

 tral proliferation of the crest cells lying close to the ectoderm. 

 Posterior to the optic vesicles on either side may be seen the 

 descending crest cells loosely packed together, wrapping around 

 the anterior and lateral surfaces of the mesoderm of the mandib- 

 ular arch. Over the dorsal border of the hind-brain region lies 

 a thin layer of crest cells which is continued upon the lateral walls 

 as a more compact band of cells. This group of crest cells de- 

 scends along the anterior border of the auditory placode to the 

 mesoderm of the hyoid arch. A distinct though more slender 

 band of crest cells along the posterior border of the auditory 

 placode approaches the mesoderm of the first branchial arch, 

 extending as far ventrally as the anterior band. Still further 



