ANATOMY OP THE MADREPOEARIAN CORAL FUNGIA. 317 



oy MetschnikofF (29), mesoblast cells are marked off in the 

 embryo of Callianira bialata before any tissues are de- 

 veloped, but after the complete separation of ectoderm from 

 endoderm. He says, further, that the case is the same in 

 Beroe and Cydippe. From the account given the segmentation 

 appears to be very peculiar. If the formation of mesoderm is 

 correctly described it would mark off the Ctenophora very 

 sharply from the remainder of the Coelenterata. Metschnikoff, 

 however, for reasons which are not quite clear to me, refuses 

 to this layer the name of mesoderm. 



No account is given of the origin of the supporting lamina 

 in the Hydrozoa, nor in the Actinaria. 



The only certain knowledge, then, that we have about the 

 origin of the jelly-like layer is that in the Alcyonarians Sym- 

 podium coralloides and Clavularia crassa the inter- 

 stitial substance is derived from the epiblast, and the cells in 

 it are epiblast cells. Thus the origin of the jelly-like support- 

 ing lamina of Coelenterata gives no direct evidence of its 

 homology with the mesoblast of the Triploblastica, but rather 

 the contrary, for the latter is, as we have seen, rather connected 

 with the hypoblast than with the epiblast. 



The characters of the supporting lamina in the Coelenterata 

 are as follows : 



In the HydromedusEe it is a fine, apparently structureless 

 membrane, interposed between ectoderm and endoderm. When 

 treated with suitable reagents it exhibits a fibrillar arrange- 

 ment ; it contains no cells. 



In the Siphonophora it is a structureless jelly-like substance. 



In the Scyphomedusse (Charybdaea) the jelly-like substance 

 is abundant, forming the bulk of the umbrella ; it contains no 

 cells, but has a fibrillar arrangement. 



In the Discomedusoe (Aurelia) the gelatinous matrix contains 

 a number of oval or star-shaped cells, anastomosing with one 

 another, and mainly derived from the hypoblast. 



In the Ctenophora it contains muscular stellate cells, mostly 

 of epiblastic origin, though some are stated by Chun to come 

 from the hypoblast. 



