BASAL PLATES IN CRINOIDEA CAMERATA 



667 



changes its introduction between the posterior radials would prob- 

 ably cause in the posterior basal in the cup of the primitive embryo. 

 Since the zone of potential weakness caused by the enlarging hind- 

 gut is evidently in the right-posterior radius of the calyx, there can 

 be but three possibilities for the enlargement of the posterior basal, 

 which are: (i) normal symmetrical enlargement; (ii) enlargement 

 through the anchylosis of a sixth basal factor appearing between 

 the posterior and right-posterior basals; and (iii) enlargement 

 through acceleration of growth in the right- or left-posterior sides. 



4 5 



Fig. 10. — Diagrams illustrating modifications in shape and position of the basals 

 and radials by the introduction of the anal plate; wavy lines indicate plate margins 

 not in contact, smooth lines indicate plate margins in contact: 1-4, stages based upon 

 the development of Antedon; 5, Tanaocrinus stage. 



i. When the anal plate came into contact with the posterior 

 basal (Fig. 10, No. 2), mutual trunkation of their apposed margins 

 would take place, perhaps to a slight degree by absorption, but 

 mainly from atrophy and broadening by lateral growth. As the 

 radials and anal came into contact (Fig. 10, No. 3), growth pres- 

 sure would cause a gradual spreading of the radial, cycle at the 

 posterior side, the lateral radials swinging outward and slightly 

 forward (Fig. 10, No. 4), with a hinging motion along the lateral 

 suture margins, until the anal plate reached its full proportionate 



