16 ROBERT TRACY JACKSON ON ECHINI. 



and mutual relations show developmental stages which it is felt, from numerous and careful 

 comparisons, can be trusted as stages in development. Stages in development are not limited 

 to the ventral border, but are seen in many types appearing progressively up to or even dorsal 

 to the mid-zone. This is especially seen in the development of the interambulacrum, where 

 new columns come in often very late in the growth of the individual, as shown in numerous 

 cases, for example, Melonechinus multi'porus (Plate 57, fig. 1). 



Localized stages in development is the principle that throughout the life of the individual, 

 stages may be found in definite parts that are comparable to the condition in the young and 

 to the adults of simpler types of the group. This principle was discovered in and applied to 

 the young plates at the dorsal portion of the interambulacrum in Palaeozoic Echini in my paper, 

 "Studies of Palaeechinoidea, " published in 1896, p. 228. It was applied to Palaeozoic and 

 Recent Echini in my paper (1899) on this principle as a new law in evolution published in the 

 Memoirs of the Boston Societj^ of Natural History, under the title, " Localized Stages in 

 Development in Plants and Animals." In this memoir the principle of localized stages in 

 development is applied to the interambulacrum, but especially to the ambulacrum, where it is 

 found that primitive characters, seen in the nascent plates at the dorsal border of the area, are 

 striking and most valuable for a comparison with the condition in the young and with simpler 

 types in the group. This principle is shown well in the development of young ambulacral 

 plates dorsally in Centrechinus (text-figs. 92, 94, pp. 106, 107) and is seen especially in the 

 family Palaeechinidae (text-fig. 237, p. 231). This principle was found to have a broad applica- 

 tion in various groups of organisms, as ophiurans, crinoids, corals, cephalopods, also plants; 

 and it has been taken up by the late lamented Professors Hyatt and Beecher, by Grabau, 

 Cushman, Ruedemann, Buckman, Hubert Lyman Clark, and Jeffrey, who found it a valuable 

 aid in studying many different groups of animals and plants. 



Senescence, which was made so much of by Professor Hyatt in his studies of cephalopods, is 

 best shown in Palaeozoic Echini by the dropping out of columns of interambulacral plates at 

 the dorsal or last built portion of the test. In the old individual of Lovenechinus 7nissouriensis 

 (Plate 42, fig. 6) columns 1 and 2 have in part dropped out dorsally, whereas in a younger 

 specimen (Plate 41, fig. 2) they continue directly to the apical disc. Melonechinus giganieus 

 (Plate 59, fig. 14) shows the dropping out of the eleventh or last added column at the dorsal 

 border of the area. Dropping out of columns is shown graphically in the old-age Melonechinus 

 indianensis (Plate 53, fig. 1) where columns 1 and 2 or even more drop out before reaching the 

 apical disc. Hyattechinus beecheri (Plate 26) shows the dropping out of columns 1 and 2 toward 

 the dorsal portion of the areas. 



Progressive types are those which show in their development to maturity the addition of 

 differential characters only, without the dropping out or disappearing of such characters, unless 

 in senescence. A good case of a purely progressive type is Palaeechinus quadriserialis (Plate 30, 



