536 HERRI CK E. WILSON 



Accelerated increase of an interpolated plate between cycles or 

 plate groups demands: (i) the decrease in size of some adjacent 

 plate or plates, as in the decrease in size of the dextro-lateral radials 

 in Pisocrinus (Fig. 7), upon enlargement of the radianal, or (2) 

 distortion of the cup. 



c) Plate division. — This process is the splitting of a plate into 

 two (perhaps more) parts, either during or after the formation of 

 the primary, formative cell group. Division of a cell group is due 

 to cell separation, and may or may not be accompanied by cell 

 division. Division of the plates when they are once formed is due 

 to the action of the absorptive, ameboid cells. Division differs 

 essentially from intercalation, in that a fundamentally distinct cell 

 group is demanded for the interpolated plate. But no matter how 

 division may take place, apparent evidences of division in the 

 plates of fossil crinoids must be very carefully investigated before 

 too much significance is attached to the opinion that division and 

 not interpolation has taken place. 



Division due to absorption is only known to occur during the 

 absorption of the supporting rods in echinoid larvae, 1 and in the 

 reduction of the radianal in Antedon. 2 Division or duplication is 

 assumed by Bather 3 in the formation of the paired, proximal inter- 

 brachials in Actinocrinidae, yet all evidence from the work on 

 crinoid larvae shows duplication and not division to be the process 

 involved. Horizontal bisection is assumed by Bather 4 in ten genera 

 of monocyclic Inadunata, not including those in which bisection 

 of the right-posterior radial only occurs. However, when it is 

 noted how closely the development and migration of the radianal 

 in the larvae of modern crinoids parallels the development and 

 migration of the radianal in the Flexibilia, 5 there is good reason to 

 believe that bisection of the right-posterior radial has not occurred, 

 but that the radianal and subradials are primary or interpolated 

 (secondary) plates. Vertical splitting seemed beyond question in 



1 Ref. 34, PP- 349-54- 



2 Ref. 27, pp. 52, 53, PI. 5, Fig. 11. 



3 Ref. 3, p. 34, fifth notice. 



4 Ref. 6, pp. 112, 144. 

 s Ref. 16, p. 332, 333. 



