396 STUDIES FOR STUDENTS 



throughout the length of the vertebral column. The second is 

 developed to the fullest extent only in the caudal portion of the 

 column and there furnishes a protection for blood vessels. These 

 arches may or may not be attached to the centrum in the adult 

 form, but the bases are the first points of ossification and the 

 rest of the vertebrae develops between them. The process is not 

 complete in all of the fishes, and the gradual completion of the 

 vertebras is of great aid in determining the position of some of 

 the fossil forms (Fig. i). 



Fig. I. Schematic view of a vertebra, n, neural arch ; c, centrum ; h haemal 

 arch ; w, notochordal canal. 



The third, and perhaps the most important of the three 

 regions of development is the formation of the fins, both paired 

 and median. Whatever may have been the original form of the 

 fish the first thing in the evolution demanded by their peculiar 

 environment must have been the development of some form of 

 keel that would not only aid the fish in its progress through the 

 water, but would enable it to maintain any desired position both 

 as to the relative depth below the surface of the water and as to 

 lateral displacement. The first step in the accomplishment of 

 this end was the development of long fins extending from the 

 head to the tail. One pair of these was developed on the 

 median dorsal and ventral lines, and persists in the dorsal, 

 caudal, and ventral fins of the existing fishes. The second pair 

 extended along each side of the body in a plane at right angles 

 to the first and divided the body into approximately equal parts 

 above and below. The paired fins, the pectoral and the ventral, 

 are supposed to be remnants of these lateral folds or fins. 



