MONOGRAPH OF THE EXISTING CRINOIDS. 



353 



proximity, mesenchyme cells become prominent, and the formation of a new 

 sacculus is begun. 



The first sacculi, five in number, appear in the Antedon larva shortly before 

 the breaking through of the vestibulum. 



762 



FIGS. 758-764. 758. DISK OF PBNTAMETKOCKINUS VARIANS. 759. DISK OF THAUMATOMETRA. BOKEALIS. 

 760. VENTRAL VIEW OF PORTION OF PINNULE OF PBRISSOMETRA SELENE, SHOWING COVERING PLATES 

 CLOSED DOWN OVER AMBULACRAL GROOVE. 761. LATERAL VIEW OF PORTION OF DRIED PINNULE OF NEO- 

 MBTRA SPINOSISSIMA, SHOWING COVERING PLATES CLOSED DOWN OVER AMBDLACRAL GROOVE. 762. TOB 

 OF GHOUSE (BONASA UMBBLLATA), SHOWING SCDTES DEVELOPED IN WINTER WHICH ARE SIMILAR TO 

 COVERING PLATES (DRAWING BY AUTHOR). 763. VENTRAL VIEW OF PORTION OF DRIED PINNULB OF 

 COMATILIA IRIDOMETRIFORMIS, SHOWING COVERING PLATES (DRAWING BY AUTHOR). 764. LATERAL VIEW 

 OF DRIED PINNULE OF COMATILIA IBIDOMETRIFORMIS, SHOWING COVERING PLATES (DRAWING BY AUTHOR). 



In describing the formation of a sacculus Reichensperger says that a limited 

 number of mesenchyme cells assemble, usually forming a small spherical cell mass. 

 These cells possess a circular nucleus with prominent nuclear bodies. The small 



