FORM REGULATION IN CERIANTHUS. 253 



mesenterial chambers were compressed and others stretched 

 according to their position on the folds. There is, however, no 

 marked difference in the length of the new tentacles on the dif- 

 ferent sides of the body, those in the region of the directives 

 being no more advanced than those in the growing region oppo- 

 site. From this figure it is very evident that the marginal ten- 

 tacles arise from the highest, i. t\, the most oral point of the 

 tentacular ridge. Moreover they arise in a single circle or row, 

 although in the normal animal they occur in about three concen- 

 tric circles. 



A longitudinal section of the body-wall at this stage is shown 

 in Fig. 19. A comparison with Fig. 18 shows marked changes. 

 The most conspicuous of these is the continued reduction in 

 thickness of the body-wall upon the ridge. The muscular layer 

 has almost or quite disappeared in this region and also between it 

 and the new tissue occupying the central region of the end, and 

 is reduced considerably in thickness for some distance aboral to 

 the ridge. At this stage then the whole oral end is closed by a thin, 

 unpigmented, translucent membrane consisting of ectoderm and 

 entoderm, but without a distinct muscular layer. The central part 

 of this membrane resulted from the growth of new tissue at the cut 

 edge, while the more distal portions forming the tentacular ridge 

 have arisen by the transformation of a part of the old body- wall 

 into tissue capable of a large amount of new growth, and of dif- 

 ferentiation into new structures. In other words, the body-wall 

 in this region has changed from its differentiated condition to 

 what is commonly called the embryonic condition. The histo- 

 logical features of this change are of great interest, but will be 

 described at another time. 



The marginal tentacles now grow rapidly, and in another day 

 (six days after the operation in summer) the oral end presents the 

 appearance shown in Fig. 20. Several changes of importance 

 have occurred since the stage of Fig. 18 : the disc is greatly ex- 

 panded, the marginal tentacles are much longer, the distinction 

 between the tissue of the old body-wall and the thin membrane 

 closing the end has disappeared completely, and finally the 

 mouth is beginning to appear as an opening between the center 

 and the periphery of the disc in the directive radius. The disc is 



