THE EPIMERE (SEGMENTAL OR VERTEBRAL PLATE) 217 



I 



Development of the thyroid gland and related hyoid cartilages. ( 1 ) Diagram- 

 matic representation of the developing thyroid of the frog in relation to the 

 hyoid cartilages in early metamorphosis; (HCP) hyoid cartilage proper, 

 (BH) basihyoid cartilage, (BB) basibranchial cartilage, (HB) hyobranchial 

 cartilage, (1) approximate initial site of the thyroid anlage, (2) approximate 

 position of the thyroid at the hindlimb bud stage, (3) approximate position of 

 the thyroid at the fully differentiated hindlimb bud stage. 



(2) Same as "1" except at later stages of metamorphosis; (4) Approximate 

 position of the thyroid at forelimb emergence, (5) approximate position of the 

 thyroid at the tail resorption stage. (D'Angelo and Charipper: 1939.) 



the two large ceratohyals. The more anterior basihyal is slow to de- 

 velop cartilage but can be identified by the U-shaped thyroid gland 

 which appears directly ventral to it. The jaws are suspended to the 

 skull by a cartilaginous bone which develops from membranes. 



Portions of the second visceral (hyoid) arch give rise to the hyoid 

 cornu. Parts of the third and fourth visceral arches (first and second 

 branchials) give rise (at the 9 mm. stage) to the plate-like hypo- 

 branchial cartilage apparatus of the adult. The four pairs of slender 

 ceratobranchials extend posteriorly from the hypobranchials. None 

 of the arches remain as such by the time of metamorphosis, the only 

 remnants being the derivatives just listed. The frog tadpole does not 

 possess true teeth, but does have ectodermally covered horny "jaws" 

 and so-called teeth. Oral papillae, appearing as teeth, constantly are 

 wearing away and bear no relation to the bones with which the 

 more functional teeth of the post-metamorphic frog will be associated. 

 At metamorphosis dermal papillae appear on the upper jaw, associ- 

 ated with the vomers, and are known as tooth germs. The overlying 

 epidermis is then involved in transforming the papillae into pseudo- 

 teeth, with a minute amount of dentin and enamel. The teeth are for 

 holding rather than for macerating living food, and never are de- 

 veloped as highly as in reptiles or mammals. 



