STROBILIZATION OF AURELIA 89 



tion of an ephyra does not refer to the transformation of the 

 original oral disc of a scyphistoma, but to the fate of the 

 segments, formed below the original oral disc. 



Formation of Manubrium.— The constriction of 

 a polyp proceeds as far as the longitudinal muscle, the ecto- 

 dermal groove interradially cutting the septum horizontally. 

 Between the interradii the endoderm is pushed inwards, but the 

 edge so formed does not reach the imaginary line taken between 

 two adjacent longitudinal muscles. This edge is concave 

 internally, so that a transverse section between two ephyra 

 rudiments would show a roughly cross-shaped gastral cavity 

 with the longer diameters lying in the perradii. About this 

 time a groove appears in the subumbral endoderm near the inner 

 edge, beginning in the perradius as a very shallow groove 

 (PI. 6, fig. 12 a) and becoming deeper passes towards the 

 septum or taeniole. The fold of endoderm thus formed pushes 

 into the mesogloea between the subumbral endoderm and 

 ectoderm (PL 6, fig. 12 &, Gr.). The groove, as it approaches the 

 taeniole, divides into two portions at its septal end. One of 

 these portions is a gutter passing rou.nd on the inside of the 

 septal muscle to meet its fellow from the other side, and the 

 other portion is a sohd plug of cells (PI. 6, fig. 12, P.) which 

 grows through the septum towards the interradius, where it 

 meets and fuses with a fellow from the other side, lying between 

 the septal muscle and the ectoderm. The muscle is thus 

 surrounded, in this manubrial region, by endoderm. The 

 portion of the plug of endoderm cells lying in the interradius 

 causes a bulging of the ectoderm at that point (PI. 6, fig. 9), 

 but the outer layer does not project all the way round. Four 

 horizontal perradial slits now^ appear in the connecting tube 

 in the same plane as the ectodermal projections (Text-fig. 2), 

 so that the gastral cavity comes into communication with the 

 outside. Finally, the slits unite across the interradial ectodermal 

 projections and the outer layer of an upper ephyra is cut 

 off from that of the next below. While this takes place, a split 

 occurs in the plug of endoderm cells connecting the two endo- 

 derm grooves (PL 6, fig. 9, S.), which now become continuous 



