808 DR. G. HERBERT fOWLER ON THE [June 15, 



practically identical with that of the ectoderm of the stomodfeum ; 

 even tlie ectodermal pigment granules, very distinct in borax- 

 carmine preparations on the body and stomodaeum, are uniformly- 

 present on the tilaments, but are not found in the undoubted 

 endoderm. 



2. Young mesenteries, Avhich have not yet become united with 

 the stoniodfeum as far down as its lower free edge, carry only a 

 thickening of obviously endoderm-cells (fig. 4) on their iree 

 margins. 



3. Mesenteries which have become united Mith the stomodseuni 

 as far down as its lower free edge (except the " directive " 

 mesenteries) carry one or other of the t\^ o types of mesenterial fila- 

 ment above described for some distance, but below this filament they 

 show a simple thickening of vacuolated endoderm-cells, of the 

 same character as they carried before they reached the lower edge 

 of the stomodoeum (fig. 4) ; as I interpret it, the ectoderm has 

 grown down along their free margins for some distance, but not as 

 yet for their whole length. 



4. The sulcus runs very much further down into the ccelenteron 

 than does any other part of the stomodseum, forming a long groove 

 of the shape indicated in fig. 6, At the point vvhei-e the ectoderm 

 of the sulcus becomes continuous laterally with the endodtsrm, the 

 histological structure is practically the same as in the filament of a 

 fei'tile mesentery (fig. 5). 



The onlv evidence, of which I know, in favour of an endoderm al 

 origin of the filament is as follows: — (1) E. B. Wilson', in his 

 studies on the development of numerous Alcyonaria, claimed to 

 have shown that the axial (dorsal) tilaments were of ectodermal, 

 the remaining six filaments of eudodermal, origin. To this one may 

 reply that Alcyonaria are not Actiniaria, although closely allied to 

 them, and that the differentiation of function, with which Wilson 

 showed that the different mesenteries were correlated, does not 

 hold good in the same shape for Actiniaria. (2) The brothers 

 Her twig - refuse to accept von Heider's suggestion of an ecto- 

 dermal origin in Cerianthus on the ground that in Sar/artia para- 

 sitica the incomplete mesenteries, which do not yet touch on the 

 stomodseum, are provided v\ith a filament similar to that of the 

 complete mesenteries. This is certainly not the case in young 

 Araclinactis, and, I may add, the filament of Sagartia j)arasitica 

 seems to be in many respects of an unusual character among 

 Actiniaria. Neither the argument from Alcyonaria nor that 

 from Sagartia appears to me to be strong enough to unseat the 

 evidence given above. If these filaments are indeed ectodermal, 

 the boundary between ectoderm and endoderm is obvious enough 

 in the digestive type of mesentery ; but in the fertile type, is 

 probably at the commencement of the vacuolated endoderm-cells, 

 as there occurs at this point what I can only describe, by borrowing 

 a phrase from geology, as an unconformability of strata. 



1 Mittheil. zool. Stat. Keapel, v. 1. 



^ Die Actinien. Jena, 187S), 8vo. (Jen. Zeitschrift, xiii.) 



