8G ANATOMY OP VERTEBRATES. 



trunk by a pair of condyles, fig. 72, e, e, is resumed. The chief steps 

 In the developement of the batrachlan skull will be premised before 

 entering upon the various modifications. In the larva of the frog, 

 fig. 42, the outer layer of the notochordal capsule expands at the 

 fore part of that vertebral basis to enclose the brain, and its appen- 

 dages, the sense-organs. The cartilage therein developed, fig. 68, 

 as the head expands, forms an occipito-petrosal mass, fig. 42, 

 16, including laterally the ear-cajisules ; it bifurcates anteriorly 

 into the ' sphenoidal arches,' wliich reunite in front of an oblong 

 hypophysial space to form a broad prefronto-vomerine mass. The 

 occipito-petrosal cartilage sends out on each side a tliick ' masto- 

 tympanic ' process, which bifurcates ; the division directed for- 

 ward and inward fig. 42, 26, is the ' pterygoid ; ' that passing 

 forward and downward is the ' hyjDotympanic' To the back part 

 is attached the hyoid cartilage, ib. 4o : to the end is attached the 

 'mandibular' cartilage, ib. so, fig. 69a, d, also called 'Meckel's 

 process.' The subsequent ossification begins partly iii the carti- 

 lage, partly in the piersistent notochordal membrane: the first 

 may be called ' chondrogenous,' the second ' sclerogenous ' bones : 

 some are disjwsed to regard the first only as ' endoskeletal,' the 

 latter as ' exoskeletal.' 



To the first category belong the neurapophyses of the occijuit, 

 exoccipitals, figs. 43 and 68, 2 ; each of which developes a ' zygapo- 

 physis ' or condyle, fig. 73, e, for tlie atlas, fig, 43, a : 

 any petrosal ossification upon the ear-capsule is a 

 growth from the cxoccipital and from the alisplie- 

 nuid, ib. 6 : the expanded disc of the ' columella ' or 

 ' stapes ' is a distinct ossicle, between 2 and 25, 

 fig. 43 ; as is also the ' hypotjTiipanic ' articulation, 

 ib. 29, for the mandible, so, 32. The neurapophyses 

 of the third segment, ' orbitosphenoid,' fio-g. 42 and 

 orti, skull Lmi 43, 10, perforated by the oiitic nerves, are ossified 

 m the cartilaginous basis, as are those ot the 

 fourth segment (prefrontals), figs. 42, 44, 68, 14, perforated by 

 the olfactory nerves ; whilst tliose of the second segment, ' ali- 

 s]ilienoids,' ib. 6, perforated by the trigeminal, longer remain 

 gristly. All the chondrogenous elements are thick bones. 



From tlie membranous basis of the skull are developed the 

 following bones, wliich are more or less lamclliform. The basi- 

 occipito-splicnoldal plate, fig. 73, w(, forms the base of the skull 

 from the condyles to tlie vonierino cartilage. The mastotym- 

 jianic, fig. 43, 25, fig. 44, 8, ■>:., fig. 68, 8," extends from "the 

 mastoid cartilage, where it is broadest, to the outside of the 



