THE SKULL. 



481 



u.l 



Frog stands, so far as development is concerned, in striking contrast 

 to the mandibular arch of the Axolotl, in spite of the obvious 

 similarity iu the arrangement of the adult parts in the two types. 



In the earliest stage it forms a simple bar in the membranous 

 mandibular arch, parallel to and very similar to the hyoid bar behind 

 (fig. 337, Mn). In the next stage observed, that is to say in Tadpoles 

 of four, five, to six lines long, an astonishing transformation has 

 taken place. The mandibular arch (fig. 838) is turned directly 

 forwards parallel to the trabecula, 

 to which it is attached in front 

 ip.pg) and behind {pd). The 

 proximal part of the arch thus 

 forms a subocular bar, and the 

 space between it and the trabe- 

 cula a subocular fenestra. In 

 front of the anterior attachment 

 it is continued forwards for a 

 short distance, and to the free 

 end of this projecting part is 

 articulated a small Meckelian 

 cartilage directed upwards {mk). 

 The Meckelian cartilage is at 

 this stage placed in front of the 

 nasal sacks, in the lower lip of 

 the suctorial mouth. The greater 

 part of the arch, parallel with 

 the trabeculae, is equivalent to 

 what has been called in the Axo- 

 lotl the quadrate, while its an- 

 terior attachment to the trabe- 

 cular is the rudiment of the 

 palato-pterygoid cartilage. The 

 posterior attachment is known as 

 the pedicle. 



The condition of the mandibular 

 ai'ch daring this and the next stage 

 (fig. 339) is very perplexing. Its 

 structure appears adapted in some 

 way to support the suciorial mouth of the Tadpole. 



Reasons have been offei'ed in a previous part of this volume for sup- 

 ])osing that the suctorial mouth of the Tadpole is probably not simply a 

 structure secondarily acquired by this larva, but is an organ inherited fi-om 

 an ancestor provided through life with a suctorial mouth. 



The question thus arises, is the peculiar modification of the mandibular 

 arch of the Tadpole an inherited or an acquired featui-e 1 



If the hrst alternative is accepted we should have to admit that the 

 mandibular arch became first of all modified in connection with the 

 suctorial mouth, before it was converted into ths jaws of the Giiatho- 



Fio. 338. Tadpole ov Common Toad, 



ONE-THIRD OF AN INCH LONG ; CRANIAL AND 

 MANDIBULAR CARTILAGES SEEN FROM ABOVE ; 

 THE PARACHORDAL CARTILAGES ARE NOT YET 



DEFINITE. (From Parker.) 



lie. notochord ; ms. muscular segments ; 

 an. auditory capsule; py. regiou of pi- 

 tuitary body ; tr. trabecula ; c.tr. coruu 

 trabeculffi ; p.pij. palatopterygoid bar ; pd. 

 pedicle ; q. quadiate condyle ; mk. Meck- 

 elian piece of mandibular arch ; s.o.f. 

 subocular fenestra ; u.l. ujjper labial car- 

 tilage. The dotted circle within the qua- 

 drate region indicates the position of the 

 internal nostril. 



B. E. II. 



31 



