ORIGIN AND SIGNIFICANCE OF SPINES 



15 



show, of course, only the longitudinal series, for at the 

 margin of the shell the radii would be of the same size and 

 age, and the spines uniform. 



The foregoing example illustrates an important principle 



19 



20 



44 



40* 



ililiiifi i 



FIGURE 19. Sector; showing in diagram the multiplication of radiating 

 lines by interpolation. The two primary radii (1,1) are the only ones continuing 

 through the whole four zones. The first zone lias 2 radii ; the second, 5 ; the third, 

 1 1 ; and the fourth, 23. 



FIGURE 20. Profiles of the spines produced on the various radii at the four 

 zones ; as indicated in the preceding figure. A, the spines on the two primary 

 radii of the first zone ; B, the spines on the second zone, showing the growth of 

 those on the two primary radii (1, 1), and the small spines on the newly interpo- 

 lated radii (2, 2, etc.) ; C, the spines on the radii in the third zone ; D, the spines 

 at the bottom of the fourth zone. The two large compound spines are on the two 

 primary radii. Their development may be traced by following them through A, 

 B, C, to D. The next three longest spines (2, 2, 2) are tricuspid, and represent 

 the stage of spine development attained by the spines on the radii which were 

 interpolated on the second zone. The next six smaller spines (3, 3, 3, etc.) are on 

 radii which were introduced on the third zone. The twelve small spines (4, 4, 4, 

 etc.) are on the radii introduced on the fourth zone. Thus there are four stages 

 of spine growth shown on the lower margin of the fourth zone, and these corre- 

 spond to the four stages exhibited by the series of spines on one of the primary 

 radii running through the four zones. 



of ontogeny ; namely, that in organisms which repeat various 

 parts during their growth, these parts will develop or pass 

 through a series of stages corresponding to the initial and 

 subsequent stages of the parts repeated. In this way struc- 



