440 



L. S. STONE 



can be found in stage 28. There is a slight thickening in the 

 ectoderm in front of and shghtly below the anterior extremity of 

 the VII placode (fig. 22). The placode of the lateral-Une ganghon 

 of VII has extended to the dorsal border of this placode, with 

 which it fuses near the dorsal portion of the hyomandibular cleft. 



,q"a3 pi. 



■mdcc- 



mda — 



23 



Fig. 23 Cross-section, showing in stage 30 the epibranchial placode of VII 

 lying below lateral-line placode of VII and also crest cells descending over mandib- 

 ular arch and dorsal portion of optic vesicles. X 37. 



Fig. 24 Frontal section through lower portions of mesoderm of mandibular, 

 hyoid, first and second branchial arches in stage 33, showing manner in which 

 arches are being completely surrounded by crest cells. X 37. 



Fig. 25 Frontal section, showing in stage 33 broad contact of ophthalmic 

 ganglion with ectoderm. X 37. 



Fig. 26 Frontal section, showing in stage 33 contact of distal portion of 

 gasserian ganglion with an ectodermal thickening at anteroventral margin of 

 supra-orbital primordium. X 37. 



The posterior extremity of the VII lateral-line placode is con- 

 tributing cells to a mass lying close upon the anterior border of 

 the auditory vesicle. Many mitotic figures can be seen in this 

 portion where the cells are separating from the placode. Above 

 the point of fusion of the placode of VII with the placode near 

 the dorsal portion of the hyomandibular cleft, the ectodermal 

 thickening becomes broadened in the anterodorsal direction 



