SOUND-TRANSMITTING APPARATUS OF CAUDATA 



351 



The youngest stage studied was an embryo 30 mm. long, in 

 which the ear capsule is completely chondrified. The sound- 

 transmitting apparatus presents a condition not met with in 

 other urodeles. The stylus is present and chondrified through- 

 out its whole length while as yet no fenestral plate is formed (fig. 

 27). A section through this region (fig. 15) shows the chondri- 



Fig. 15 Transection through the fenestra vestibuli of an embryo Amphiuma 

 means 30 mm. long. Ac, arteria carotis interna: CI., canalis lateralis; Col., 

 stylus columellas (columella); Ec, ear capsule; Fe.m., fenestral membrane; 

 Vp.L, vena petrosolateralis. 



fied stylus lying along the fenestral membrane, which here, as at 

 all levels, entirely lacks cartilage. The progress of chondrifi- 

 cation, so far as direct observation is concerned, remains un- 

 known, since stages of the proper age were not available. The 

 stylus extends only a few microns along the fenestra. Two 

 series of embryos 33 mm. long (which vary as regards advance 

 in development) give a clue as to one mode of plate formation, 



