Plate IV. 



Fig. 35. Ambystoma punctatiim, adult. Section farther caudad than Fig. 

 30. Operculum, opercular muscle and recessus perilymphaticus are shown. 



Fig. 36. Chondrotus tenebrosus, transformation period. Section through 

 stilus columell?e and the cephalic end of the developing operculum. 



Fig. 37. Ambystoma punctatum, transforming. Horizontal section show- 

 ing columella, stilus, squamoso-columellar ligament, the developing operculum 

 and the M. opercularis attached in front to the opercular plate and behind 

 to the suprascapula. 



Fig. 38. Chondrotus tenebrosus, transformation period. Section farther 

 caudad than Fig. 36. It shows the columella. The operculum is partially cut 

 out from the floor of the ear capsule. 



Fig. 39. Ambystoma punctatum, embryo 13-14 mm. in length. The colu- 

 mella consists of a mass of cells against the membrane of the fenestra and 

 connected by a strand of dense tissue with the cells vipon the prominence of 

 the lateral semicircular canal. Above the columellar proton is the vena 

 petroso-lateralis ; below, the carotid artery. 



