480 L. S. STONE 



farther ventrally, very small rudiments of the second, third, and 

 fourth branchial cartilages appear, which, after they approach 

 each other at their ventral extremities, become completely lost 

 and do not approach in the midline the cartilages of the normal 

 side. The hyoid cartilage is completely absent. The first and 

 second ceratobranchials and the ceratohyoid are absent on the 

 operated side (fig. 74). The first and second basibranchials are 

 present. • 



A specimen in which all the crest cells were included in the 

 operation along with the upper half of the neural tube was killed 

 eleven days after operation, and it shows many deficiencies in the 

 visceral skeleton. Very small first, second, and third epibran- 

 chial cartilages are found on the operated side (fig. 75); they 

 soon disappear as the sections are followed ventrally. The quad- 

 rate in this and other sections is smaller than on the normal 

 side. The hyohyal, ceratohyal, and all the ceratobranchial carti- 

 lages are entirely absent. The mandible is smaller on the right 

 side. The first basibranchial cartilage is present but small and 

 lies toward the right of the midline, while the anlage of the second 

 basibranchial is apparently normal. No change can be seen in 

 the anterior portions of the trabeculae. 



A similar attempt was made in another specimen, but due to 

 incomplete removal of crest cells in the branchial region regenera- 

 tion had taken place. However, seventeen days after operation 

 it showed externallj^ a diminution in size of the right side of the 

 lower jaw. A section at a level with the lower border of the 

 ear and the optic nerve shows a remnant of the quadrate very 

 small (fig. 76) when compared with figure 77, a similar level on 

 the left side. At the level of the lower border of the right eye the 

 quadrate cartilage appears again for a short distance and extends 

 toward the articular end of the mandible. As the sections are 

 followed farther ventrally, the mandibular cartilage on the right 

 side is seen to be smaller than on the normal side. 



Another specimen was operated upon at stage 26 and killed 

 twenty-one days later. At this stage it is impossible to remove 

 successfully all of the crest cells which have migrated down over 

 the mesoderm of the mandibular arch. The external gills and 



