CCELENTERATA. 



157 



thirty-two masses of transparent protoplasm form an hypoblastic 

 vesicle. The embryo at this stage is shewn in optical section in 

 fig. 71. 



The epiblastic vesicle now grows rapidly, while the hypo- 

 blastic vesicle remains nearly passive and becomes somewhat 

 lens-shaped. At one point its wall comes in close contact with 

 the epiblast. Elsewhere a wide cavity is developed between the 

 two vesicles which becomes filled with gelatinous tissue. At this 

 period cilia appear on the surface, and the larva becomes a planula. 



The succeeding changes lead rapidly to the formation of a 

 typical Medusa. Where the epiblast and 

 hypoblast are in contact the former layer 

 becomes thickened and forms a disc-shaped 

 structure. The centre of this becomes 

 somewhat protuberant, fuses with the hy- 

 poblast and then becomes perforated to 

 form the mouth (fig. 72 <?). The edge of 

 the disc forms a thickened ridge, the 

 rudiment of the velum (v), which is en- 

 tirely formed of epiblast. At its edge six 

 tentacles (t] arise, into which are con- 

 tinued solid prolongations of the wall of the now somewhat 

 hexagonal gastric chamber. The hypoblastic axes of the tenta- 

 cles soon lose their connection with the gastric wall. 



Up to this time the larva _ == ___ = _^_ 



has retained a more or less 

 spherical form, and the cavity 

 on the under side of the 

 umbrella has not yet become 

 developed. The latter now 

 becomes established by the 

 whole disc assuming a vault- 

 ed form with the concavity 

 directed downwards. The 

 lining of the cavity so formed 

 is derived from the epiblast 

 of the disc already spoken of. 



The exact mode of formation of the gastrovascular canals has not been 

 worked out. It has however been established by the researches of the 



FIG. 71. EMBRYO OF 

 GERYONIA AFTER DELAMI- 

 NATION. (After Fol.) 



ep. epiblast; hy. hypo- 

 blast. 



FIG. 72. OPTICAL SECTION THROUGH 

 THE ORAL POLE OF GERYONIA AFTER THE 



APPEARANCE OF THE GELATINOUS TISSUE 



OF THE DISC. (After Fol.) 



o. mouth; v. velum; /. tentacle. 

 The shaded part represents the gelati- 

 nous tissue. 



