VERTEBRA AND RIBS 



41 



bears a pair of transverse processes. The ribs of frogs and toads are 

 always short, and it is a question whether the cartilages on the tips of 

 the diapophyses of Rana, etc., are ribs or epiphyses. 



AMNIOTA. — The frequent occurrence of amphicoelous vertebraj 

 in reptiles, birds and mammals, at least in development, points to the 

 origin of the group from an amphicoelous ancestor, but there are 

 many points of vertebral ontogeny on which definite knowledge is 

 lacking, as are also detailed comparisons with the neural and haemal 

 parts of Ichthyopsida. It is evident, however, that many genera 

 have the same cranial and caudal half-sclerotomes (figs. 17, 48), and 

 that these play the same parts in form- 

 ing a vertebra with caudal elements in 

 front, cranial from the next following 

 somite, behind. 



The notochord is very small, its 

 sheath greatly reduced, consisting of a 

 single elastica, except in Sphenodon 

 embryos where there is a thin interna 

 and a slightly thicker externa. Until 

 the appearance of skeletogenous tissue 

 the diameter of the notochord is the 

 same throughout, although it may 

 have an undulating course. The 

 neuropophysial cartilages are usually 

 first to appear, the haemapophyses 

 in the tail appearing at the same time 

 in some forms. In the trunk hjrpo- 

 chordal bars, extending beneath the 

 chorda and up on either side, are 

 formed in procartilage. These, appar- 

 ently, are haemal elements comparable 



to the hypocentra arcaha of some Ichthyopsida (p. 36). With 

 chondriiication these bars join cartilages from the neural elements, 

 forming rings around the notochord. In a few lizards intercentra 

 arise between the successive neural arches, uniting soon with the 

 principal parts. 



The succeeding stages differ somewhat in Sauropsida and mammals. In 

 Sphenodon and geckoes (which lack close vertebral articulation) the notochord 

 persists between the centra, and the vertebras are amphicoelous, as in several 



Fig. 48. — Early stages in develop- 

 ment of cervical vertebrae of cow 

 (Froriep, '86). A, transverse of 

 embryo 8.8 mm. body length; B, 

 sagittal through basal plate and 

 cervicals I and 2 of 25 mm. embryo, 

 c, centra of atlas and epistropheus; b, 

 basal plate of cranium; g, spinal 

 ganglion; h, hypochordal bar; m*. 

 fifth myotome; w, notochord. 



