CRANIAL GANGLIA OF AMBLYSTOMA 475 



An attempt to eliminate the neural-crest cells was made in 

 several ways. By means of scissors and a fine spear-point needle 

 the neural-crest cells were scraped away from the wall of the 

 neural canal and, according to age, also carefully lifted off the 

 mesoderm upon which they had migrated. The ectoderm was 

 then turned back into its normal position, where it was weighted 

 with glass rods and silver wires in the usual manner and allowed 

 to heal. This treatment on one side only did not eliminate the 

 possibility of the neural crest either from the sources of its origin 

 on the operated side or from cells of similar groups wandering over 

 from the unoperated side. Therefore, a series of embryos im- 

 mediately after this operation was put into a cool chamber to 

 retard development, and ten or twelve hours later the normal 

 left side was similarly treated. In this type of double operation, 

 however, the absence of normally developed crest cells makes a 

 comparative study difficult. 



A study of a large number of the first type of operation in which 

 the crest cells were scraped away from one side only shows a 

 large percentage in which regeneration of the crest cells had taken 

 place, while many of the specimens which had undergone the 

 double operation proved to be very oedematous in the gill region 

 and unfit for study. Many of these individuals also showed 

 complete regeneration of the neural crest. 



In order to insure against the regeneration of the neural crest, 

 a more extensive operation was performed. The ectoderm was 

 flexed back in the manner already described and, after the neural 

 crest had been carefully scraped away, the upper half of the 

 neural tube on the right side was removed from the anterior 

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

 above the eye. The number of cases available was considerably 

 diminished by the high mortality attending the presence of such 

 a large wound — only fifteen out of one hundred and fifty sur- 

 viving. The high mortality was partially overcome by 

 diminishing the area of operation by one-half. The ectoderm was 

 flexed back upon the middorsal line so as to expose the crest cells 

 over the branchial region. The crest cells along with the upper 

 half of the neural tube from the anterior border of the third 



