THE EDWAEDSIA-STAGE OF LEBEUIflA. 2S9 



many cells in thickness, and its margin is strongly ciliated and 

 nucleated. 



In sections for some distance below the middle of the larva the 

 mesenteries are all of the same size, and there is a large central 

 cavity, but towards the aboral region the cavity is again nearly 

 obliterated. The details here represented indicate the stages 

 followed during the progress of disintegration. The vertical 

 section in fig. 21 will assist in making the relations described 

 Sb little clearer. 



Prom these details it is evident that the only actual dis- 

 integration as yet is that involving the central tissue internal to 

 the two circumferential sinuses, and extending to the aboral 

 region and some distance within the oesophageal region. In the 

 expelled larvse the process, like that of the formation of the oral 

 aperture, was set up immediately after liberation, and must 

 have proceeded rapidly ; for in only three specimens was I 

 able to obtain the stage in which the centre is still solid and the 

 diverticula distinct throughout. On the other hand, it is well on 

 the way in the non-extruded larvae, which are still devoid of any 

 external opening. 



E.emains of the primary solid condition continue to be repre- 

 sented for some time peripherally, by an enormously thickened 

 ridge of parenchymatous tissue, connected with the columnar 

 boundary of each radial chamber, and also by the ridge of similar 

 tissue on each face of the first pair of mesenteries (fig. 19). 

 And in none of the polyps has the disorganization proceeded so 

 far that the central cavity extends to the aboral region ; but the 

 process has progressed further at the oral extremity, whence the 

 hollow tentacles arise. 



All the preparations reveal, from the beginning, only a thin 

 epithelium on the three pairs of shorter mesenteries, except 

 towards their connection with the column and with the archen- 

 teron, where the layer joins that of the column-wall. 



As growth continues the columnar intermesenterial ridges 

 must also become further diminished until the unilaminar epi- 

 thelium characteristic of the adult polyp is obtained throughout, 

 but the larvae only present stages towards such a condition. 

 There is every indication that it is the limiting layer of the larval 

 spaces, and not the vacuolated tissue, which persists and con- 



