No. 556] 



ALPHEUS HYATT 



203 



repeat the characters seen in youth and in the adults of 

 more primitive types. Such localized stages are shown 

 by many trees and other plants. In the oak, ash and 

 hickory, suckers from the base of the tree have simple 

 forms of leaves, comparable to those seen in young seed- 

 lings. Beneath the flower (rose, peony), at the tips of 

 branches (hickory, sassafras) and in diseased or feeble 

 growths (tulip-tree, red cedar) leaves often occur which 

 in simplicity of character are comparable to those of 

 seedlings or more primitive species in the group. 

 Localized stages occur also in herbaceous plants as 

 shown by Cushman. 13 



Amongst animals localized stages are shown where 

 during growth there is an addition of similar parts as 

 the plates in echinoderms, septa in cephalopods and in 

 the developing zooids of colonies of corals and, accord- 

 ing to Ruedemann, in Graptolites. In these types the 

 parts as added present stages which are comparable to 

 stages seen in the ontogenesis of the individual as a 

 whole. In Echini new plates are added to the corona 

 immediately below the oculars, and at this region 

 throughout life the ambulacral plates are of a simple 

 character, whereas the older earlier formed plates during 

 their individual development may have taken on com- 

 plex characters, for example, in Centrechinus {Diadema) 

 ambulacral plates are compound, but close to the oculars 

 are simple. In the Paheozoic family of the Palaeechin- 

 idae, the ambulacrum at the equator, or midzone, has from 

 two to twelve columns of plates in each area, but in those 

 genera with many columns there are only two columns 

 dorsally in the area where new plates are added. In 

 crinoids, in which the arms have the plates arranged in 

 a biserial manner (Encrinus, PlaUjcrinus) as shown by 

 Ghrabau, 14 a uniserial arrangement exists at the tips 



