XO. 8 INTERRELATIONSHIPS OF THE CETACEA WINGE 3 



dorsal vertebras also. Its powerful muscles, which have their origin 

 in part on the sacral and dorsal vertebrae, and are also in connection 

 with the muscles of the back, widen their place along the spinal 

 column and stimulate the vertebrae to increase in bulk. The sacral 

 and lumbar vertebras come to resemble the largest, most anterior 

 caudal vertebrae exactly, apart from the lower arches. On a few of 

 the hindmost thoracic vertebrae, which in the ancestral forms are 

 without, or as good as without, transverse processes, there grow out 

 powerful transverse processes on a line with and similar to those 

 of the lumbar vertebras (parapophyses, apparently corresponding to 

 the lower section of the double, rib-bearing " transverse process " of 

 the anterior thoracic vertebras, which supports the rib's capitulum ; 

 but in reality they most probably correspond to the upper and lower 

 sections combined), and on their tip they eventually bear the attach- 

 ment surface for the rib. Apparently this surface may be either for 

 the attachment of the tuberculum or of the capitulum or of the two 

 coalesced, but in reality it is perhaps always for the two combined 

 (or, more correctly, not separated). The transverse processes of the 

 anterior thoracic vertebrae (diapophyses, the upper portion of the 

 double "transverse processes"), which in the beginning are quite 

 short, may eventually grow long, pushing far out to the side the 

 articular surface for the rib's tuberculum which they bear at their 

 extremity. On all the thoracic vertebrae the spinous processes finally 

 become high and strong. 



The hind limb atrophies completely, and disappears. At length 

 only quite insignificant parts of its skeleton are found, hidden deep 

 under the skin, finally in the form of a mere little rod-shaped bone, a 

 remnant of the pelvis.^ In small embryos the hind limb can, however, 

 still be distinguished externally. 



The disappearance of the hind limb has a great influence on the 

 vertebral column. No longer does a pelvic bone come in contact with 

 any of the vertebrae. In consequence the sacral vertebras completely 

 lose their peculiarities : their characteristic strength, their mutual firm 

 connection, their robust transverse processes with flattened areas for 

 the hip bones. They are formed exactly like the adjacent dorsal and 

 caudal vertebras. The movements of the spinal column become 

 changed in character. Bending of the column in the vertical plane, 

 which depends especially on the movements of the hind legs, is 

 reduced or abandoned, and as a result the differences in slant — 

 forward or backward — of the spinous processes as good as disappear. 



