CRANIAL GANGLIA OF AMBLYSTOMA 457 



eye. The ectodermal thickening over the dorsal extremity of 

 the hyomandibular cleft as well as the longitudinal ectodermal 

 thickening seems to have no significance except in so far as it 

 is the result of a change in contour of the underlying tissue. As 

 early as stage 25 (fig. 3), however, the placodes of lateral-hne 

 ganglia of VII and X can be identified, while all the epibranchial 

 placodes can be located at stage 26-27 (fig. 4). The primordia 

 of all the groups of the lateral-line sense organs are established 

 at stage 35+ (fig. 8), and out of this pattern the sense organs are 

 laid down in the skin. 



EXPERIMENTAL 



All the operations were confined to the right side of the em- 

 bryos and consist of two groups: a) extirpating the placodes in 

 the ectoderm, followed by grafting into the wound indifferent 

 ectoderm taken from another animal of the same age and, b) re- 

 moving the neural crest either by scraping it loose or by removing 

 it along with the upper half of the neural canal from the anterior 

 border of the third somite to the anterior extremity of the brain 

 above the eye. 



Where indifferent ectoderm was transplanted into denuded 

 areas the grafts were removed from animals which had previously 

 been stained in the jelly capsule with Nile-blue sulphate (Det- 

 wiler, '17). This made possible daily observations upon the 

 early growth and position of the implanted tissue. 



The mortality of operated animals was lowered by keeping 

 them during the three or four days following the operation in a 

 glass chamber, around which cool tap-water ranging from 11° to 

 15°C. was constantly running. This retarded wound healing 

 at first, but no disturbances were observed to affect the results 

 of the experiments. 



In stage 25 the auditory placode becomes marked externally 

 as a small round pigmented spot a little below an elevation in the 

 region of the hind-brain (fig. 3). This may be used almost 

 invariably as a landmark in orienting operations upon the placo- 

 dal regions in this and early embryos. Since there is no external 

 evidence of the pigmented spot previous to this stage, the eleva- 



