86 



INTRODUCTION. 



retrogression is shown in the following series of 

 figures of Bufo vulgaris (after Thiele). 



Fig. 32. 



Development and retrogression of the snbbuccal apparatus in 



Bufo vulgaris. 



It appears as a crescentic groove very early, before 

 any other organ, and disappears, after having under- 

 gone various changes, in the beginning of the tadpole 

 period. Its fullest development coincides with that of 

 the external gills. It is single and discoid, with a 

 crescentic groove, in Discoglossus; single and Y-shaped 

 in Pelodytes and Pelobates ; single and V-shaped or 

 crescentic in Bvfo ; and paired, forming two small 

 disks, in Bombinator, Bana, and Hyla. These disks 

 are close together in Bombinator, widely separated in 

 Ht/la and Bana temporaria and agilis, whilst they are 

 connected by a fine transverse ridge in Bana escu- 

 lenta, which thus affords a link between the single and 

 paired types of holders. Hyla differs from all others 

 in the disks shifting forwards so as to be, in the 

 later stages, situated on a level with the mouth. 

 The condition in Discoglossus appears to be the most 

 primitive, from which those shown by Bombinator, 

 Bana, and Hyla on the one hand, Pelobates and Bufo 

 on the other hand, may be derived. The following 



