94 LEWIS R. GARY. 



growing tissue as is always found in the new mesenteries in the 

 forms that I have studied. Comparing these two figures with 

 those of the series shown in my Figs. 10 to 14, Pis. II. and III., 

 it seems very evident that all of the mesenteries shown in Carl- 

 gren's Fig. 7, Taf. IX., are old ones which have come over in 

 the fragment from the parent individual and which will never 

 come to be a part of the permanent system of mesenteries of the 

 actinian arising from the laceration embryo. In the younger 

 embryo shown in my series of figures just mentioned the first 

 formed pair of mesenteries has already acquired their mesenteric 

 filaments, which are as yet, of course, in an undifferentiated 

 condition; but nevertheless clearly distinguishable as mesenteric 

 filaments. 



In the embryo from w r hich my series of Figs. 16 to 18 was 

 taken, which has at the level of the stomodeum only four more 

 mesenteries than that from which Carlgren's figures under discus- 

 sion were taken, eight of the new mesenteries, at the level at which 

 Fig. 1 8 was taken, are provided with filaments of which those on 

 the first pair of mesenteries are already trilobed, while several of 

 the others show the first steps in the process of differentiation 

 by which the trilobed condition is reached. 



(c) The Development of the Mesenteric Filaments. 



The question of the origin of the mesenteric filaments among 

 the anthozoa has been discussed to considerable length by most 

 workers on the development of these forms. With the exception 

 of E. B. Wilson, 1884, all of the previous investigators have 

 confined themselves to a consideration of the embryonic develop- 

 ment. The above-named investigator worked with the polyps 

 arising by budding in several alcyonarians, but not with embryos 

 of the same forms. 



E. B. Wilson (loc. tit.} reached the conclusion that in the 

 Alcyonaria the dorsal directive filaments arise as downgrowths 

 of the ectoderm of the stomodeum, while all the other filaments 

 arise through the differentiation of the endoderm at the free 

 border of the mesentery. He would homologize the dorsal 

 filaments of the Alcyonaria with the lateral ciliated bands (flim- 

 merstriefen) of the trilobed actinian filament. The filaments 



