CRANIAL GANGLIA OF AMBLYSTOMA 453 



further medially than in the previous stage described. The 

 occipital group is still represented by four primordia, which are 

 becoming elongated as they approach each other. The dorsal 

 body line has progressed posteriorly as far as the twelfth somite, 

 along the path of which have appeared the anlagen of a varying 

 number of sense organs. The midbody line has progressed 

 caudally as far as the twenty-seventh somite. As it reaches a 

 point just above the anal region it curves dorsally a short dis- 

 tance, then caudally where it travels along the upper border of 

 the somites of the tail. Along its course have developed many 

 anlagen of sense organs. The ventral body Une has descended 

 behind the external gills anterior to the limb region to the ventral 

 border of the mesoderm of the limb bud where it begins to bend 

 caudally. 



Stage 39 



Neural crest. Since the previous stage the aggregations of 

 neural crest have become still more compact (fig. 35) and may now 

 be regarded as the procartilage of the visceral skeleton. Only a 

 few crest cells can be identified outside of the procartilage region, 

 although, as has already been suggested, many of the loose cells 

 around the maxillary and mandibular groups may be of crest-cell 

 origin and may be forming connective tissue. The procartilage 

 still contains many small yolk granules and also many mitotic 

 figures. The position of the mesoderm of the branchial arches 

 is now occupied by blood vessels and anlagen of branchial muscles, 

 while in the position of the mandibular and hyoid mesoderm are 

 the anlagen of mandibular and hyoid muscles. 



Stage L2 



Neural crest. In stages 40-41, of which no sections have been 

 figured, the only perceptible changes in the procartilage are the 

 gradual disappearance of yolk granules, the decrease in the 

 number of mitotic figures, and the steady increase in the density 

 of the cellular arrangement. In stage 42 true cartilage is laid 

 down as well-defined, solid bars in the positions formerly occupied 

 by the procartilage (fig. 36). The yolk granules have entirely 

 disappeared from the branchial cartilages. 



