CRANIAL GANGLIA OF AMBLYSTOMA 429 



hind-brain. A cap of crest cells, whose base extends as a wedge 

 between the fused neural folds, lies in the region of the mid-brain. 

 These cells are represented by well-formed nuclei surrounded by 

 a pigmented cytoplasm in which he fine, closely packed yolk 

 granules which are similar in size to those of the neural canal. 

 The mass of cells lies close to the ectoderm and extends laterally 

 a short distance over the walls of the mid-brain. 0\'er the region 

 of the hind-brain a similar small cap can be seen lying in the 

 same relation to the ectoderm and brain, except that its base does 

 not appear in this plane of section to be wedged between the 

 fused folds. 



Placodes. Figure 1 shows an area over the dorsal border of the 

 eye, which represents the position of a very shght thickening of 

 the ectoderm where cells are given off later to the ophthalmic 

 ganghon. It may be said here that throughout the different stages 

 studied, this ectodermal thickening never acquires a thickness 

 comparable to that of other placodes. Many times in the dis- 

 sections the area was thinner than the surrounding ectoderm 

 because of the fact that its cells adhered to the loose ganghonic 

 mass of cells forming the ophthalmic ganghon. This thickening 

 may be called the ophthalmic placode. 



Below the anterior extremity of the longitudinal ridge lies a 

 small ectodermal thickening in the dorsal extremity of the hyo- 

 mandibular cleft, which can be detected at this stage. This 

 small ectodermal thickening takes on the contour of the cleft 

 at this point, giving it approximately a crescent shape. In the 

 position of the anterior portion of this thickening there later 

 arises the primordium of the infra-orbital group of lateral-line 

 sense organs, while in the position of the posterior portion there 

 later arises the hyomandibular group. 



Stage 23 



Neural crest. This stage shows a decided ventral growth of the 

 neural crest over the lateral walls of the brain (fig. 2). Above 

 and posterior to the dorsal border of the eye an extensive pro- 

 liferation has taken place, carrying the neural crest downward 

 toward the primitive mandibular arch. Figure 14 shows above 



