686 THE SKELETAL SYSTEM 



tebrae possess hemal arches and the trunk region without hemal arches but 

 with ribs. The amphibia begin to show a third division, the cervical area or 

 anterior portion of the trunk region in which the vertebrae do not possess ribs. 

 This area is Umited to one vertebra, the axis. In the amphibia, also, a sacral 

 region begins to make its appearance. It is only slightly differentiated in water- 

 abiding forms but well developed in the Anura. The caudal vertebral area in the 

 Anura generally is fused to form the coccyx or urostyle. The reptilian vertebral 

 column manifests great variability in the different orders. The turtles show cer- 

 vical, trunk, and tail regions, with the trunk vertebrae fused with the bony plates 

 of the carapace. In snakes, a short cervical area, a greatly elongated trunk 

 region, and a caudal area are present. Some of the snakes possess the largest 

 number of vertebrae among verterbates, the number reaching several hundreds. 

 Sacral vertebrae are absent in snakes. The lizards and crocodilians show condi- 

 tions closely resembling the amphibia. In the birds, caudal, synsacral, thoracic, 

 and cervical regions are present, while, in mammals, cervical, thoracic, 

 lumbar, sacral, and caudal regions exist. 



d) Ribs. Ribs are not found in cyclostomatous fishes. In the gnathostomes, 

 two types of ribs may be present: 



( 1 ) dorsal ribs and 



(2) ventral or pleural ribs. 



Fig. 321 — (Continued) 



the vertebra appears to arise from two basidorsal and two interdorsal arcualia as indi- 

 cated in fig. 321, G. The origin of the basidorsal and interdorsal vertebral rudiments 

 from the sclerotomic mesenchyme are shown in figure 321, J-M. The vertebrae are of 

 the acoelous (aniphiplatvan) type (fig. 321, S). The chevron bones and hemal arches 

 in the tail region of many mammals represent basiventral elements. Fig. 321, M-O, shows 

 the rib outgrowths from the developing vertebrae. Observe centers of ossification in the 

 vertebra in fig. 321, O. 



Fig. 321, A, presents a lateral view of the so-called arcualia in relation to the notochord 

 and the myosepta (myocommata). According to this theory of the development of the 

 vertebrae, the arcualia form the main rudiments from which future vertebrae arise. (B) 

 The adult frog vertebrae showing probable contributions of arcualia. (C and C) Prob- 

 able contributions of the arcuaha to trunk and tail vertebrae of Sqiialus acanthias. (D) 

 The adult vertebrae of Necturiis maculosiis. (E) The role played by the arcualia in 

 forming the axial supporting structure in Acipenser sturio. (Redrawn and modified from 

 Goodrich, Vertebrate Craniata, 1909.) (F) The composite origin of the vertebra in 

 the bird. (Redrawn from Piiper, 1928. Phil. Trans. Series B, 216.) (G) Probable con- 

 tributions of the arcualia to vertebra formation in man. (H) Probable contributions 

 of the arcualia in the formation of trunk and caudal vertebrae in Ainia catva. (1) Same 

 for the teleost. Conodon nohilis. (J-L) The origin and early development of the 

 sclerotomic mesenchyme in the mammal. (M) shows vertebral and costal development 

 in a 15-mm. pig embryo. (N) presents vertebral and costal development in a human 

 embryo of 1 1 mm. The vertebral and rib rudiments are in the mesenchymal stage at 

 this period. (Redrawn from Bardeen, 1910. Keibel and Mall, Vol. I, Human Embryology, 

 Lippincott, Phila.) (O) is a drawing of developing vertebra in the 22-mm. opossum 

 embryo. (P, Q, R, and S) are diagrams of amphicoelous, procoelous, opisthocoelous 

 and slightly biconcave amphiplatyan (acoelous) vertebrae. (Redrawn and modified from 

 Kingsley, '25.) 



