476 L. S. STONE 



somite to just in front of the ear were removed. In cases where 

 it was advantageous the crest cells upon the mesoderm of the 

 hyoid arch were included among the tissue that was removed. 

 The number of recoveries from this kind of operation was far in 

 excess of the number of deaths. 



The neural crest over the mandibular and hyoid regions was 

 removed along with the upper half of the neural tube from the 

 posterior border of the ear to above the anterior border of the eye. 

 The mortality in this type of operation far exceeded the recov- 

 eries. The ectoderm lying above the optic vesicles is very narrow, 

 and when it is cut loose along its ventral border it soon shrinks 

 toward the middorsal line, so that when the neural canal is re- 

 moved the anterior extremity of the wound is often poorly 

 covered. The healing is often incomplete at the anterior ex- 

 tremity of the head. The hole remaining, however small, is 

 responsible for a large percentage of the deaths. 



Among the group of embryos from which the neural crest has 

 been removed by scraping there appear many cases which show 

 varying degrees in the diminution of the size of the external 

 gills. A very few showed shght deficiencies in the branchial 

 cartilages. 



The most favorable results were obtained from the specimens 

 from which the neural tube was removed along with the neural 

 crest. Among those cases in which this method was used to 

 eliminate the crest cells over the branchial region and occasionally 

 over the hyoid region there are recorded a number of specimens 

 killed at various stages in development after the operation. 



Case 1. The operation was done at about stage 26 and the 

 embryo was killed three days later. It had not developed very 

 rapidly as it remained in the cool chamber up to the time of 

 killing. Frontal sections through the mesoderm of the arches on 

 the normal side show that the crest cells are completely surround- 

 ing them. A section at the level of the middle of the optic cups 

 (fig. 68) shows a complete absence of crest cells on the mesoderm 

 of the second branchial arch on the operated side. On the 

 median side of the mesoderm of the first branchial arch only a 

 few crest cells appear, which are continued in a few sections fur- 



