96 LEWIS R. GARY. 



(loc. cit.} for Lebrunia, it is recorded that the filaments appear 

 first on the upper part of the mesenteries at the level of the free 

 border of the stomodeum, and usually there can be observed 

 a direct continuity between the ectodermal lining of the stomo- 

 deum and the filaments from the first appearance of the latter. 



The mesenteries acquire their filaments in the same order 

 as that in which they arose, so that the dorso-laterals would 

 first show these structures. In Fig. 12, PI. II., this pair of 

 mesenteries alone shows the beginning of the filaments. Farther 

 up in this series of sections the tissue forming the filaments is 

 directly continued into the lining of the stomodeum. In Fig. 9, 

 PI. II., a longitudinal section through a young laceration 

 embryo, the filament extends for a short distance down along the 

 free border of the mesentery, and here there is no histological 

 differentiation to mark the lower limits of the stomodeum. Its 

 actual limit can, however, be determined by a comparison w r ith 

 the other side of the figure where the section passed between 

 two mesenteries so that the border of the stomodeum hangs free 

 "in the gastroccel. H. V. Wilson (loc. cit.) has pointed out that 

 in Manicina the filaments of all the first twelve mesenteries, save 

 the first pair, arise from a fold of the ectoderm- at the free border 

 of the stomodeum which bends up and outward into the gas- 

 trocoel pointing toward the oral disc. In this manner the fila- 

 ment begins to grow down the mesentery before the edge of the 

 latter has in its upper part come into contact with the stomodeum. 



In the development of laceration pieces of Aiptasia there is the 

 same folding backward of the wall of the stomodeum at an early 

 stage in the development (Fig. 8), so that the course of the 

 development of the filaments in this form follows that already 

 described for Manicina. 



In an older specimen, Fig. 18, PI. II., the filaments of the 

 first pair of mesenteries have assumed the trilobed form charac- 

 teristic of the filaments of adult Aiptasias, while the remainder of 

 the mesenteries have at this level either simple filaments or 

 none at all. It is thus apparent that the change in the structure 

 of the filaments follows the same order as does the appearance of 

 the filaments themselves. 



In this transition from the simple to the tri-lobed structure 



