564 



INTRODUCTION TO AMNIOTA 



[CH. 



attached. Therefore it is roughly true to say that the centrum of 

 Amniota corresponds to the intervertebral cartilage of Urodela and 

 vice versa. The cartilaginous basiventral of Amniota is an in- 

 sufficient support for the rib when this becomes heavy, and hence 

 not only is a transverse process (diapophysis) developed from 

 the neural arch which joins a "tubercular" process on the rib, 

 and thus forms an additional support for it, but the proximal 



FIG. 278. Two stages in the development of the front part of the vertebral 

 column of an Amniote (the Lizard). 



A. Stage in which hasidorsals, basiventrals and intercalaries are separate. 



B. Stage of the completed vertebral column the first four vertebrae are 

 shown. 



1. Basidorsal 2. Notochord. 3. Basiventral. 4. Ventral inter- 

 calary, i. Atlas vertebra formed by union of basidorsals and basi- 

 ventrals. ii. Axis vertebra formed from the second intercalary with 

 the first intercalary attached to it as odontoid process. in. Third 



vertebra. iv. Fourth vertebra. Dotting, cross-hatching, etc. as in 

 Figs. 232, 242, 254 and 267. 



end or head of the rib is broadened and becomes attached to the 

 centre (ventral intercalaries) before and behind it, and sometimes 

 even becomes shifted so as to be completely attached to one of 

 them. Even in this case, however, the rib betrays its origin by 

 being attached to one end of the centrum, not as in Fish and 

 Urodela to the middle of the centrum. 



All Amniota agree in possessing a neck. This is a region of 

 the body supported by a varying number of vertebrae which inter- 

 venes between the head and the heart. No trace of a neck is found 



