CRANIAL GANGLIA OF AMBLYSTOMA 



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the longitudinal ectodermal ridge small portions of the neural 

 crest descending upon the lateral walls of the hind-brain con- 

 nected dorsally by a cap of cells still wedged tightly between the 

 ectoderm and brain. 



Posterior to the mandibular arch are the two prolongations of 

 the neural crest which appeared in stage 21. Approaching the 



Fig. 13 Frontal section passing through longitudinal ectodermal ridge 

 and anterior and posterior extremities of layer of neural crest in stage 21, showing 

 origin of crest cells from dorsal and median portions of neural folds. X 37. 



Fig. 14 Cross-section passing through longitudinal ectodermal ridge and 

 ectodermal thickening in dorsal portion of hyomandibular cleft in stage 23, 

 showing neural crest over dorsal portion of neural tube and portions of neural 

 crest descending to lateral walls of neural tube. X 37. 



Fig. 15 Cross-section through upper half of optic vesicles in stage 25, showing 

 crest cells descending along sides of neural tube and over mesoderm of mandibular 

 arches and over anterior portion of right optic vesicle. X 37. 



first somite is an additional proliferation which later covers the 

 mesoderm of the second, third, and fourth branchial arches. 

 In this stage the neural crest has extended posteriorly (fig. 2) 

 to a position above the anterior half of the fourth somite. 



Placodes. The ophthalmic placode lies in relatively the same 

 position as in stage 21. Its indefinite borders outline a slightly 



