248 COMPARATIVE ANATOMY 



centrally with the endoderm of the manubrium and peripherally 

 with the endoderm of the ring-canal. This sheet, known 

 as the endoderm-lamella, extends right round the medusa, 

 and the radial canals and ring-canal are nothing more than 

 channels hollowed out in it. These relations will perhaps be 

 better understood after a consideration of the development of 

 a medusa as a bud on the blastostyle as represented by the 

 diagrams, fig. 53, A E. In A the bud is shown as a simple 

 hollow outgrowth of the blastostyle wall. In B the ectoderm 

 cells at the extremity of the bud have multiplied and separated 

 into ec y an external epithelial layer continuous with the rest of 

 the ectoderm, and bn, a solid mass of cells pressed close 

 against the sac-like outgrowth of the endoderm. This solid 

 mass of cells is known as the bell-nucleus. In C the bell- 

 nucleus has increased in size, and a cavity has appeared in its 

 centre. As a consequence of its increase the centre of the 

 endodermic outgrowth has been further pushed in, whilst its 

 periphery embraces the bell-nucleus, so that the latter structure 

 lies like a ball in the mouth of a double- walled cup of endoderm. 

 In the next figure D the cavity of the bell-nucleus has increased 

 greatly in size, and the ectoderm cells surrounding it have 

 arranged themselves as an epithelial layer against the mesoglcea 

 covering the walls of the endodermic cup. The last-named 

 has correspondingly increased in size, and forms a deep cup 

 with double walls. The space between the double-walls is a 

 part of the gastrovascular cavity, and is still in free communica- 

 tion with the cavity of the blastostyle. A further structure has 

 made its appearance at the bottom of the cup in the shape of 

 a simple finger-like outgrowth of endoderm pushing before it 

 the mesoglcea and the ectoderm of the bell - nucleus. The 

 essential parts of the medusa are now established. The cavity 

 of the bell-nucleus is the sub-umbrellar cavity. The walls of 

 the endodermic cup, appearing as two horns in longitudinal 

 section, are the endoderm lamella, still double, and enclosing a 

 cavity reaching all round the walls of the bell or umbrella. 

 The outgrowth at the bottom of the cup is the primordium of 

 the manubrium. In the next figure JE the medusa is nearly 

 complete. The tissues at the extremity of the bud have 

 thinned out and finally broken through, converting the cavity 

 of the bell-nucleus into the sub-umbrellar cavity. The manu- 

 brium has increased in size, and projects freely into the sub- 



