256 RITTER AND CROCKER 



The parts involved at this early time are : The ectoderm of 

 the floor of the new ray, and intimately associated with this, the 

 radial nerve band ; the radial perihasmal canals ; the radial 

 water tube ; and the coelom of the new ray. 



The relations of these are made intelligible by an examina- 

 tion of figs. 5, 9, 10, II, 12 and 13. 



Fig. 5 is an oral view of the interradius in which a new ray is 

 coming in sight, three pairs of tube feet having alread}'- become 

 visible. The orientation here will be readily understood by 

 noting the peristome, -per, and two tube feet amh, of the old 

 rays between which the new one is forming. Fig. 9 is a vertical 

 radial section through an entire interradius in which a new ray, 

 somewhat younger than the one shown in fig. 5, is forming. 

 The point x corresponds in the two figures, and its closeness to 

 the circular nerve band in fig. 9 will be noted. This moves Way, 

 either by growth or actual splitting, as development progresses, 

 as seen by reference to fig. 5 ; but until even a considerably 

 later time than this, the pocket, into which the new tube-feet 

 project, as shown in fig. 9, persists, it having pushed forward as 

 rapidly as the point x has. 



Fig. 10 is taken from a frontal section of an interradius in 

 which the newly developing ray is still younger than the one 

 shown in fig. 5. The section is low down and consequently 

 cuts the interradial mass ventral to the ray pocket, the position 

 of which is indicated by dotted outline. 



The very narrow chink here, m.amf.^ by which the pocket 

 communicates with the exterior, will be noted, and indicates 

 clearly how, so far as the ectoderm and radial nerve band of 

 the new ray are concerned, the development began. The nar- 

 row ectodermic invagination which began the pocket was at 

 the very outer edge of the circular nerve band, «.r., and some 

 of the latter was taken in, so to speak, to become the radial 

 nerve band of the new ray. 



The method by which the new radial water tube and the new 

 ccelom originated are also obvious from fig. 9. The new water 

 canal, r.tu.v.y in particular, brings out the strong curvature 

 which the new ray has at this time. The terminal tentacle, 

 ten.^ though already free at its tip, is still held by its base in 



