478 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



their connection witli t!i(> stoniodanun they are tipped with a tissue of like nature. Iiiereasint;' in 

 extent, it is continued as the mesenterial tilament along the edge of the tirst pair of mesenteries, 

 almost as far as their termination at the aboral end of the larvse, ceasing on one mesentery a little 

 in advance of the other (<■/. also Pis. XVIII. XXV). 



At this early stage the mesenterial tilament is not sharply marked otl', except histologically, 

 from the rest of the mesenterial epithelium. Its numerous luiclei stain brilliantly in borax 

 carmine, and structurally it is indistinguishable from the stomodival ectoderm. This resem- 

 blance, combined with the absolute continuity of the two at the ct)mmencoment of the tilaments, 

 would seem to remove all d()ul)t that the two — stomodjeal ectoderm and the mesenterial filaments — 

 are of one and the same origin. 



Hut the conclusion becomes less certain wiieii the incomplete mesenteries are taken into 

 accoiuit. for along their free edge is a tissue of exactly similar nature; yet the mesenteries are in 

 i\() way connected with the stomoda'um. and there is no apparent means by which the incipient 

 tilament can have lieen in unity with its ectoderm. 



The early appearance of the tilaments on the second pair of mesenteries is represented in 

 fig. 11"2, from a section taken a little above the termination of the stomoda-um. At first onh' 

 one mesenter}' displayed any marginal modification, but in the figure the tilament has appeared 

 on the other member of the pair, while lielow it is as strongly developed on both as on the first 

 pair of mesenteries, but disappears in advance of the filament of the latter. In some of the 

 sections coming below that represented in tig. 112 there is the feeblest hint of the tilament on 

 the mesenteries of the third pair, which scarcely extend beyond the endodermal lining. 



It is manifest therefore that the filaments on the second pair of mesenteries originate 

 quite independently' of any connection with the stomodicum and of the reflected ectoderm. 

 From a study of the condition.s in both the third and the second pairs it is inconceivable how at 

 any earlier stage, say before the middle eml)ryonic tissues had broken down, that any such 

 connection could have been established. The refiected ectoderm passes backwardly ))ut a short 

 distance along the ccelomic surface of the stomoda?um. and there is no possibility of its working- 

 its way u'pward, across what I'epresents the disk, and then downward along the free edge of the 

 mesentery. The presence of filaments on the second pair of mesenteries before union with the 

 stomoda'uin would imply that a similar developiiK'ut may also take place on the third pair of 

 mesenteries before their union, and sections reveal that sucli actually occurs. It is also manifest 

 from tile sections, that before the union of the mesenteries with the stomodanmi is efi'ected. there 

 is no means l)y which the free margin of the former can have come into contact with the reflected 

 ectoderm. 



From the conditions represented in the larva> of corals generally, the conclusion is reached 

 that the mesenterial tilaments may originate independently of any connection with the stomoda?al 

 ectoilerm. and may therefore ))e assumed to be endodermal. 



The tilaments on both the complete and incomplete mesenteries at the early stages of lai\al 

 development present but little histological differentiation, except in the case of the tilament on 

 the tirst mesenterial pair. They consist mainly of supporting cells, and stain nmch more deeplj- 

 than the ordinary endodermal lining, but gland cells and nematoc^'sts ai"e scarcely determinable. 

 Similar details are often presented by the edge of adult mesenteries, which always remain free 

 from the stomoda?um, showing that in the orders beyontl the primary the filaments originate in 

 tiie same manner. In these cases the tilaments may never become fully developed, but remain 

 in an incipient or rudimentary condition. This is illustrated l>y the mesenteries of OrhivfUa 

 (icrajwra. The six pairs of mesenteries of the tii'st cycle are complete, and filaments are well 

 developed below; similar filaments appear on the mesenteries of the second cycle, which fail to 

 reach the stomoda?um; but on the third cycle of twelve pairs the mesenteries are merely tipped 

 with a deeply-staining tissue, which is indistinguishable from that on the tilaments of larvae. 

 Further, some mesenteries bear incipient tilaments only in their upper course, while the organs 

 are fully developed below. C'ladocora ariuscula also aflords similar illustrative examples (PI. 

 VII). The edge of a mesentery in its upper course is rej)resented in tig. 57 r/, and the filament is 

 seen to be quite rudimentary; beU)w the stomoda'al region, however, the tilament on the same 



