CRANIAL GANGLIA OF AMBLYSTOMA 



439 



the first somite. All of the crest cells are then confined to the 

 broad band which now descends over the mesoderm of the 

 second, third, and fourth branchial arches. 



Placodes. The ophthalmic placode is still an elongated thick- 

 ening (fig. 20), in which may be seen mitotic figures. Its pos- 

 terior border is in close contact with the neural crest. It gives 



i 



21 



.gas pi 



epi S 





22 



^-S°»p^ 



ipi 



19 



Fig. 19 Cross-section, showing in stage 30 the manner in which crest cells 

 are completely surrounding mesoderm of hyoid and mandibular arches. Also 

 shows ectodermal thickening in dorsal portion of hyobranchial cleft containing 

 epibranchial placode of IX. X 37. 



Fig. 20 Frontal section of elongated ophthalmic placode in stage 30. X 37. 



Fig. 21 Frontal section of stage 30, showing point of contact of gasserian 

 ganglion near its distal end with a small ectodermal thickening at anterior 

 extremity of VII lateral-line placode. X 37. 



Fig. 22 Frontal section, showing in stage 28 a small ectodermal thickening 

 (gasserian placode) slightly below the anterior extremity of VII lateral-line plac- 

 ode. X 37. 



the appearance of contributing placodal cells to the ophthalmic 

 ganglion. 



Near the anterior extremity of the elongated placode of VII 

 there is a slight thickening in the ectoderm. A few loose cells 

 are connected with the inner side of the thickening (fig. 21). 

 This area may be a small placode concerned with the gasserian 

 portion of V. The only similar area in stages earlier than this 



