332 H. D. REED 



connected. From embryos 10 mm. long to the time chondrifi- 

 cation of the columellar rod begins, it midergoes a decided re- 

 duction in diameter. It, however, retains its connection with 

 the vestige of the hyomandibular cleft through a sheet of fascia 

 w^hich spreads out fan-shaped for attachment to the lining of the 

 oral cavity. The dense rod of undifferentiated cells, so appar- 

 ent in embryos of 10 mm., gives rise to both the columella and 

 the sheet of fascia just mentioned. The two are distinct only 

 as regards the more segregated nature of the cells which form 

 the columellar proton. 



In specimens 17 mm. long only slight changes have taken 

 place over those conditions which obtain in the 15-mm. stage. 

 The columellar cord is slightly thicker and lies at times a little 

 closer to the fenestral membrane. Nothing, by way of struc- 

 ture or relations at this period of development denotes any 

 morphological difference between the sound-transmitting appa- 

 ratus of Spelerpes and that of Amblystoma. 



Larvae from 19 mm. to 23 mm. in length are important as 

 showing the first step in the formation of the fenestral plate, 

 although considered apart from older stages, they are without 

 significance and indeed might prove misleading. The whole 

 history of development and morphology of these structures is 

 e\ddent only after the consideration of a complete and carefully 

 selected series of stages. In the 19-mm. stage the fenestral lips, 

 in the ventrocephalic extent, through proliferation and growth 

 of their own cells, extend out into the fenestral membrane in the 

 form of a triangle, the apex reaching the level of the columellar 

 cord of cells. This growth of cells becomes the isthmus fenes- 

 tralis which, as mentioned above, serves as the connection be- 

 tween the ear capsule and the definitive fenestral plate, and, as 

 will be pointed out presently, constitutes the proton of the plate 

 itself (figs. 23 and 24). 



From this stage to those 21 mm. long the only noticeable 

 changes are in the continued growi^h of the cells of the isthmus 

 into the membrane and the elaboration of a cartilaginous ma- 

 trix. The columellar cord has not yet begun to chondrify and 

 there is no indication of a fusion of the two elements. The posi- 



