BASAL PLATES IN CRINOIDEA CAMERATA 535 



plate in a closed cycle is asymmetrical, trie growth of one of the 

 laterally adjacent plates will be greater than that of the other, and 

 will, on completion of the reduction, occupy the entire area of 

 the missing plate. This process is shown nearing completion in 

 the reduction of the antero-lateral radials in Catillocrinus, 1 and the 

 consequent enlargement of the left posterior and anterior radials, 

 and in Mycocrinus 2 in the reduction of the left anterior and dextro- 

 lateral radials. In Pisocrinus the principle is diagrammatically 

 shown in both basal and radial cycles, where, by the reduction of 

 two basals and three radials, three basals, two radials, and the 

 radianal are greatly enlarged (Fig. 7, Nos. 1-4). 



* # $& <# 



1234 



Fig. 7. — Diagrams illustrating reduction and compensating growth in Pisocrinus: 

 1, hypothetical, ancestral stage; 2-4, based upon specimens in the University of Chi- 

 cago collection; x = position of first anal plate. 



b) Enlargement and compensating reduction. — Plate enlarge- 

 ment, as we have seen, is due to the activity of the ameboid calcif- 

 erous cells. In normal, symmetrical development the growth of 

 the young plates is for a short time more rapid than that of the 

 body wall, but upon plate contact the increased enlargement of 

 both plates and body wall is theoretically balanced. If, however, a 

 plate increases more rapidly than the adjacent plates, and is not 

 controlled by the inhibiting influence of symmetrical development, 

 and its accelerated growth is not compensated for by growth of 

 the body wall, this growth must be compensated for in some other 

 manner. Accelerated lateral increase of this type in a cycle plate 

 demands then either (1) the decrease in diameter of some adjacent 

 plate or plates in the same cycle or (2) the increase in diameter of 

 some adjacent plate or plates in the apposed cycle and (3) distor- 

 tion of the horizontal outline of the calyx or various combinations 

 of the first two of these secondary developments may occur. 



1 Ref. 6, p. 149, Fig. LXII. 2 Ref. 28, p. no, PL 7, Fig. 4. 



