182 ALTERNATIONS OF GENERATIONS. 



brane, ALquorea Mitrocoma. Such layers when on the outer 

 side of the membrane separating epiblast and hypoblast are 

 undoubtedly epiblastic in origin, but in some cases amongst the 

 Actinozoa they adjoin the hypoblast, and are very probably 

 derived from this layer. 



The origin of the gelatinous tissue is still involved in much 

 obscurity. 



It originates as a homogeneous layer between epiblast and 

 hypoblast, which in the Hydromedusae never becomes cellular 

 though traversed by elastic fibres. 



In the Acraspeda it contains anastomosing cells in the main 

 apparently (Glaus) derived from the hypoblast, and in the Cteno- 

 phora it is richly supplied with muscular stellate cells for the 

 most part of epiblastic origin, though some are stated by Chun 

 to come from the hypoblast. On the whole it seems probable, 

 that the gelatinous tissue may be regarded as a product of both 

 layers ; and there are some grounds for thinking that it is an 

 immense development of the membrane always interposed be- 

 tween the two primary layers. It must however be borne in mind 

 that a membrane, regarded by the Hertwigs as the equivalent of 

 the ordinary membrane between the epiblast and hypoblast, can 

 be usually demonstrated on both surfaces of the gelatinous 

 tissues in Medusae. The skeletogenous layer of the Actinozoa 

 is probably the morphological homologue of the gelatinous 

 tissue ; but the evidence we have is on the whole in favour of the 

 connective-tissue cells it contains being epiblastic in origin. It 

 gives rise to the skeleton of the Hexacoralla, to the spicular 

 skeleton of Alcyonium, the axial skeleton of Corallium, and the 

 skeleton of the Helioporidae and Tubiporidae. 



Alternations of generations. 



Alternation of generations is of common occurrence amongst 

 the Hydrozoa, and something analogous to it has been found to 

 take place in Fungia amongst the Actinozoa. It is not known 

 to occur in the Ctenophora. 



The chief interest of its occurrence amongst the Hydro- 

 medusae and Siphonophora is the fact that its origin can be 



