CRANIAL GANGLIA OF AMBLYSTOMA 435 



posteriorly, a somewhat broader proliferation, covering the an- 

 terior border of the first somite, descends ventrally on the gill 

 mesoderm to the level of the crest cells on the first branchial 

 arch. The neural crest continues posteriorly above the dorsal 

 border of the somites along the dorsolateral border of the neural 

 canal to a position in the middorsal line above the middle of the 

 third somite. 



Placodes. The longitudinal ectodermal ridge has disappeared 

 at this stage, due to the rapidly descending neural crest and the 

 growth of the visceral arches. The extent of the area of the 

 ophthalmic placode is about the same as in stage 23. The 

 thickening in the dorsal portion of the hyomandibular cleft has 

 changed but little in size. A placode approximately triangular 

 in shape, situated anterior to the auditory placode, can be seen 

 rapidly approaching the hj^omandibular cleft from above. This 

 is the placode which gives rise to a large part of the lateral-line 

 ganglion of VII by contributing large numbers of placodal cells 

 from its posterior extremity (fig. 16). The epibranchial placode 

 of VII, judging from its position in subsequent stages, is probably 

 situated just below the posterior border of this placode. This 

 probable position is circumscribed by dotted line in figure 3. On 

 the posterodorsal border of the hyomandibular cleft is a slight 

 thickening of the ectoderm, which is identified as the epibranchial 

 placode of IX. However, no placodal cells are splitting off from 

 the ectodermal thickening to form a ganglion (fig. 16). In the 

 dorsal portion of the first branchial cleft there is a small thicken- 

 ing of the ectoderm, which is possibly the position of an epi- 

 branchial placode, which gives rise to part of the sensory visceral 

 ganglion of X, although no cells are splitting off at this stage. 

 The possible position of the other epibranchial placode of X is 

 also circumscribed by dotted line in the figure. Posterior to the 

 auditory placode and anterior to the first somite, a slight thick- 

 ening in the ectoderm is identified as the placode which gives rise 

 to most of the vagus lateral-line ganglion. 



