CRANIAL GANGLIA OF AMBLYSTOMA 427 



cartilages are concerned, the morphological findings in this paper 

 correspond very closely to the observations recorded in the pres- 

 ent study of Ambl3^stoma. In Plethodon the manner of the 

 migration and disposition of the neural crest upon the branchial 

 arches is similar to that described by Piatt ('97). In addition to 

 contributing to the ganglionic portions of V, VII, IX, and X, the 

 neural crest is described as forming the greater portion of the 

 mesenchyme in the ventral part of the head and in the branchial 

 region, the anterior portions of the trabeculae, Meckel's cartilage, 

 the palatoquadrate bar, and all the branchial cartilages except 

 the second basibranchial. 



MATERIAL, METHODS, AND NORMAL DEVELOPMENT 



All the experiments have been made upon embryos of Amblys- 

 toma punctatum in stages ranging from 21 to 27 as shown in 

 figures 1 to 4. The technique emploj^ed in the operation is 

 similar to that already described by Harrison ('18). The special 

 types of operation employed in this investigation will be subse- 

 quently described under separate sections. 



In order that the experiments may be more clearly interpreted, 

 a description of the stages in the normal development of the 

 neural crest and of certain placodes will first be given. This 

 description was obtained chiefly from a series of dissections sup- 

 ported in the more minute details by serial sections. In order to 

 accomplish the dissections, embryos which had been preserved in 

 the ordinary corrosive sublimate acetic mixture were placed in a 

 5 per cent aqueous solution of nitric acid, where they were kept 

 from twelve to twenty-four hours for the purpose of softening the 

 brittle ectoderm and at the same time rendering it pliable. They 

 were then placed under the binocular microscope, and by means 

 of a small pair of operating scissors and suitable needles a con- 

 tinuous incision was made up the middorsal line and down the 

 midventral line. The two halves of ectoderm were carefully 

 removed, stained lightly with haematoxylin, cleared in the usual 

 manner, and mounted in damar on glass sHdes. After such 

 treatment the positions of ectodermal thickenings can be observed 

 accurately. 



