LOCALIZED STAGES IN DEVELOPMENT. 131 



genital plate. Two pores were the commonest variation, three pores were not rare how- 

 ever, and one specimen had two pores in two genital plates and three pores, four pores, 

 and seven pores respectively in the other three genital plates. In the 244 specimens 

 examined, 12 had more or less than the typical number of four plates in the periproct. 

 Three or five plates were the commonest variation and are well known, but two specimens 

 showed an increase over this number, one having nine plates in the periproct. In 175 

 specimens of Stroiif/yloceiitrofxs drohaclilcnsis from Massachusetts Bay only two speci- 

 mens had accessory pores. In one of these specimens there are two pores in the 

 madreporic plate, in tlie other there are two pores in a genital plate; but as a concur- 

 rent variation there are two madreporic plates. In the Palaeozoic geiiera Rhoechinus, 

 Oligoporus, and Melonites the genital plates have typically more than one pore, vary- 

 ing from two to five ; but in Melonites, where best known, usually three or four. 

 Tlie occurrence of accessory pores in Arbacia may be the expression of an atavic ten- 

 dency, but is more probably to be considered a monstrosit}', which, liowever, is paiallel 

 to the normal feature of an ancient but distinct group of Echini. 



From the above observations on Strongylocentrotus and Arbacia it is seen that there 

 is a distinct ontogenesis of interambulacral plates, in which localized parts we get in a gen- 

 eral way a repetition of tlie ontogenesis which may be traced in following a series of 

 specimens from the young to the adult. As a corollary, it is seen that, passing from the 

 dorsal border of the interambulacrum downward, or ventrall}', a progressive series of 

 stages may be traced in progressively older plates of one individual, which are broadly 

 comparable to the stages seen in a progressive series of individuals from the very young 

 to the adult. 



Palaeozoic Echini. In studies of 3Idonites multiportis ivom the Subcarboniferous 

 of St. Louis, Missouri, I have shown ('96) by tabulations of interambulacral areas that 

 these areas may differ in the same individual in regard to the number of columns of 

 plates. In one specimen tabulated (1. c, p. 165) there are eight columns in two areas 

 and nine columns in the otiier three areas. In another specimen there are eight col- 

 umns in two areas and nine in two areas, the fifth area being incomplete dorsally. Still 

 a third specimen has seven columns in one area and eight in another. The same feature 

 is shown well in a fine specimen of Olujojiorus missouriensifi Jackson from the Subcar- 

 boniferous of Webb City, Missouri. In this specimen, there are five columns of plates 

 in two areas and six in three areas. I have shown ('96) that in the development of 

 Palaeozoic Echini there is a progressive increase in the number of columns of plates 

 until the full number is attained ; also, that primitive species have fewer columns than 

 more specialized species. We have, therefore, in this radial differentiation in an adult, 

 an expression of localized radial differentiation, in which some radii arc structurally nmre 

 primitive, while others are more differentiated in the line of evolution of the group. 



