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



practically identical with that of the ectoderm of the stomodseum ; 

 even the ectodermal pigment granules, very distinct in borax- 

 carmine preparations on the body and stomodseum, are 'uniformly 

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

 endoderm. 



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

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

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

 margins. 



3. Mesenteries which have become united with the stomodaeum 

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

 mesenteries) carry one or other of the two 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 stomodseum (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 coelenteron 

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

 of the shape indicated in fig. 6. At the point where the ectoderm 

 of the sulcus becomes continuous laterally with the endoderm, the 

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

 fertile mesentery (fig. 5). 



The only 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) filaments were of ectodermal, 

 the remaining six filaments of endodermal, 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 

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

 dermal origin in Cerianthus on the ground that in Sagartia para- 

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

 stomodeeum, are provided with a filament similar to that of the 

 complete mesenteries. This is certainly not the case in young 

 Araclmactis, and, I may add, the filament of Sagartia parasitica 

 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. Neapel, v. 1. 



2 Die Actinien. Jena, 1879, 8vo. (Jen. Zeitschrift, xiii.) 



