THE LATER DEVELOPMENT OF THE FROG 217 



of the cranium, and through the elongation of the quadrate, 

 also some distance laterally from the cranial wall (Fig. 82). 



The infrarostral cartilages, which very early fuse across the 

 mid-line forming the apex of the lower jaw, also elongate at 

 this time, and now fuse with the Meckelian cartilages as the 

 mento-Meckelian cartilages. Later on each becomes bony and 

 fuses with the dentary, the chief membrane bone of the lower 

 jaw (see below). A small median element between the infra- 

 rostrals fuses with them. 



ch 



FIG. 83. Hyoid and branchial arches of a 29 mm. larva of R. fusca. Ventral 

 view. After Gaupp, from Ziegler. 66, Basibranchial (first), or copula; bh, 

 basihyal; ch, ceratohyal; ho, hypobranchial plate; 1-4, first to fourth cerato- 

 branchials. 



The annulus tympanicus surrounding the tympanic mem- 

 brane of the frog, forms as an outgrowth of the quadrate carti- 

 lage; it becomes separate and gradually extends to the surface 

 of the head, forming first a crescentic, then a circular cartilage, 

 and later bone, long after the completion of metamorphosis 

 (Fig. 82, B). 



The development of the hyoid and branchial arches may 

 conveniently be described together. These all appear first as 

 paired rods of dense tissue, lying in the corresponding visceral 

 arches. The hyoid arch forms about the same time as the 

 mandibular, the first branchial just after hatching, the second 

 branchial at 9-10 mm., and the third and fourth branchials 

 shortly after. 



