492 L. S. STONE 



that the positions of the proliferating lateral ectoderm correspond 

 to placodal regions and since the migration of crest cells soon 

 produces a scarcity of these cells in the dorsal region of the neural 

 canal and an abundance of crest cells in the region where placodal 

 cells are given off, a condition is brought about which would lead 

 to a confusion as to the origin of the wandering 'mesectoderm.' 

 Aside from the different interpretation in the origin of the 'mes- 

 ectoderm' in Amblystoma the manner of the formation of the 

 branchial cartilages with the exception of the second basibran- 

 chial agrees with the description which Piatt ('97) gives of the 

 branchial cartilages in Necturus. This is fully in accord with 

 Landacre ('21). The branchial cartilages with the exception of 

 the second basibranchial have been conclusively shown to have 

 their origin in the neural crest. At the time when this skeleton 

 begins to take on a cartilaginous appearance the first basibran- 

 chial extends a short distance posteriorly from the attachment 

 of the ceratobranchial cartilages. This condition is somewhat 

 misleading for it gives the appearance that the second basi- 

 branchial is a posterior outgrowth from the first basibranchial. 

 However, such is not the case, for a study of embryos about stage 

 42 conclusively shows that the second basibranchial is formed out 

 of mesoderm, near the anterior wall of the pericardial chamber 

 and that this cartilage retains large mesodermal yolk granules 

 for a long time after the branchial skeleton from the neural crest 

 has lost all of its yolk granules (fig. 37). Amblystoma in this 

 respect agrees with Landacre's ('21) description of Plethodon. 

 The experimental results show that some of the neural crest is 

 incorporated in the connective tissue of the external gills as well 

 as in the balancer, as Harrison ('21) has shown. How much more 

 of the connective tissue in the branchial region is formed from the 

 crest cells is impossible to determine at this time. 



In the case of the mandibular and quadrate cartilages the 

 experimental results, as already stated, did not show as conclu- 

 sively as the findings from the study of normal embryos that 

 they are derived from the neural crest, because of the difficulty in 

 eliminating the crest cells from the trigeminal region. Never- 

 theless, the results do show that there was a decided diminution 



