CRANIAL GANGLIA OF AMBLYSTOMA 445 



A few loose crest cells appear to join the ventral extremities of 

 the second, third, and fourth branchial arches. 



A new feature in the rapidly migrating crest cells now appears. 

 They begin to accumulate along the median side of the arches 

 where many mitotic figures can be seen, especially in the lower 

 halves of the latter. The typical condition is seen in the section 

 on the median side of the hyoid arch. At this level the mandib- 

 ular arch is not completely surrounded by crest cells on its 

 anterior border as it is farther ventrally. If the sections are 

 studied still farther ventrally, it can be seen that the crest cells 

 are continuous across the ventral extremities of the mesoderm of 

 the hyoid arches in front of the pericardium and across the ven- 

 tral extremities of the mandibular and maxillary processes be- 

 hind and in front of the stomadaeum. If the sections are studied 

 farther dorsally, all the branchial arches show, along their ventral 

 halves, the condition similar to that of the hyoid arch (fig. 30). 



Placodes. No longer is there any contact of the ophthalmic 

 ganglion with the ectoderm, although its anterior end lies close 

 to it. All traces of an ectodermal thickening in this region have 

 disappeared, leaving the ectoderm the same thickness as before 

 its appearance. 



The primordium of the supra-orbital line of sense organs now 

 appears as a prominent ovoid placode whose anterior end is 

 directed anterodorsally (fig. 7). Its posterior border is joined 

 by the incipient ramus ophthalmicus superficialis VII, while its 

 ventral border is connected with the ectodermal thickening in 

 the dorsal portion of the hyomandibular cleft. Along the course 

 of the anterior extremity of the ectodermal thickening, which 

 was shown in stage 33, there now descends near the posterior 

 border of the optic vesicle a well-defined primordium of the infra- 

 orbital line of sense organs. Along the posterior extremity of 

 the ectodermal thickening is the small primordium of the hyo- 

 mandibular group of sense organs which is connected to the incip- 

 ient ramus buccalis VII. 



The epibranchial placode of VII lies just posterior to the pri- 

 mordium of the hyomandibular group of sense organs and upon 

 the posterior portion of the hyomandibular cleft (tig. 7). It has 



