CRANIAL GANGLIA OF AMBLYSTOMA 467 



4. Removal of the infra-orhital, hyomandibular, ventral hijomandib- 

 ular and mandibular primordia. A number of trials were made 

 upon embryos under stage 26 to remove separately, if possible, 

 the primordia of the infra-orbital and hyomandibular groups of 

 lateral-line sense organs on the side of the head by excising small 

 segments of ectoderm behind the upper posterior border of the 

 optic vesicle. These attempts proved to be fruitless, for in all 

 cases the ingrafted tissue failed to suppress their development 

 and complete regeneration of the two primordia took place. 

 However, among the cases where an area of ectoderm including 

 placodes of the VII ganglion was removed there appears one 

 case, operated upon at stage 26, in which the hyomandibular 

 group of sense organs is entirely absent, although the infra-orbital 

 group is intact as well as the groups of sense organs on the lower 

 side of the jaw (fi^. 49). The area removed (fig. 50) also includes 

 the epibranchial, the preauditory, and supra-orbital placodes. 

 This seems to indicate that the infra-orbital and hyomandibular 

 groups have separate primordia. 



A few operations were made in an attempt to remove the pri- 

 mordia of the sense organs on the under side of the jaw. The 

 most successful case appears in an individual operated upon at 

 stage 25. The area removed is shown in figure 51, one day after 

 operation, and it includes a small amount of ectoderm anterior and 

 posterior to the lower portion of the hyomandibular cleft. 

 Several days after operation (fig. 52) the area of the transplanted ' 

 indifferent ectoderm may be seen at a lower level than its original 

 position and the migration of its posterior portion extends into 

 the anteroventral border of the first external gill. The balancer 

 is lacking on this side. Twenty-one days after operation this 

 individual reveals no ventral hyomandibular group of sense organs 

 on the right side of the ventral portion of the lower jaw (fig. 53). 

 However, there is a remnant of the mandibular group in the form 

 of a few small sense organs along the anterolateral border of the 

 lower jaw. This may possibly be an indication of the existence of 

 a separate primordium for the mandibular group of sense organs. 



5. Removal of the epibranchial placode of VII and surrounding 

 ectoderm. The area of the extirpation in this type of operation 



