LOCALIZED STAGES IN DEVELOPMENT. 



133 



over, by taking the distal spines on these 23 rows, there result the same stages of spine 

 fJevelopinent as shown in the longitudinal series along any primitive plication (Fig. 13) . 

 For example, in Spondylus, Figs. I'l and 13, two radial spinose plications, 1, 1, originate 

 near the umbo. During growth the continuations of these plications diverge, and the 



A 



32 3 13^3 1323 



Spoxdvlus sp. 



2. Sector showing diagramuiatically the multiplication of radiating lines by interpolation. The first zone A, has 2 

 radii ; the second zone B, has .5 radii ; the third zone C, ha.s 11 radii ; and the fourth zone D, has 23 radii. 



13. Profiles of spines produced on the various radii at the four zones, as indicated in Fig. 12. A, the .spines on the 

 two primary ladii of the first zone ; B, the spines on the second zone, showing the gi'owth of spines on the two primary radii 

 (1, 1) and the small spines on the newly interpolated radii (2, 2, 2); C, the spines on the radii in the third zone; D, the 

 .spines at the bottom of the fourth zone. In zone D the two large compound spines (1,1) are on the .two primary radii. 

 Their development may be traced by following them through A, B, C, to D. The next 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 B. The next smaller spines (3, 3, 3, 3, 3, 3) are on radii which were introduced in the third zone C. The twelve 

 small spines (4, 4, etc.) are on the radii introduced on the fourth zone D. Thus, there are four stages of spine development 

 shown on the lower margin of the fourth zone, and these correspond to the four stages seen in the series of spines on one 

 of the primary radii running through the four zones. C. E. Beecher, Amer. journ. .sci.. 1808. 



spines become more elaborate, as shown in Fig. 13, zones A-D. The primary plications, 

 with their progressively added spinose modifications, may be taken as the type series. 

 During growth new radial plications (2-1) originate by interpolation. At their incep- 

 tion, zones B, C, D, the spines on new plications resemble the spines of the primary 

 plications at their inception, and during radial growth the spines take on added char- 

 acters in the same way as did those of the primary plications. This increase by interpola- 

 tion may be repeated several times in the growth of the individual, with the same result. 

 The localized ontogenesis of radial ornamentation is seen only in the method of develop- 

 ment by interpolation. Species in which the radii are all introduced at an early stage 



