MARCH: GERM CELLS OF CORYMORPHA. 251 



and there is a tendency for the cells to become arranged 

 in two layers, a deeper layer next to the entoderm, containing 

 the germ cells, and an outer layer, composed only of somatic 

 cells. 



A later stage in the development of the medusa bud is 

 shown in figure 6. There is a continued increase in size, and 

 the "glockenkern" has spread out to form an inverted cup- 

 shaped mass around a central elevation of entodermal cells. 

 This central elevation indicates the general shape of the 

 manubrium, and that part around the outside, where the 

 entoderm turns sharply back upon itself, will form the um- 

 brella of the medusa. The cells of the "glockenkern" have 

 now separated into two distinct layers. The outer layer, com- 

 posed only of somatic cells, lies closely pressed against the ecto- 

 dermal layer surrounding the whole bud. The inner layer, 

 composed of some somatic cells and all of the germ cells 

 which have migrated into the "glockenkern" at this time, lies 

 close to the entodermal cells beneath. This deeper layer is 

 the ectoderm of the manubrium, which serves as a germ gland 

 in Corymorpha. The outer layer of somatic cells becomes 

 the lining of the umbrella. 



As the medusa bud develops, the split between the two 

 layers of cells in the "glockenkern" completely separates the 

 manubrium from the outer capsule of the bud (figure 7). At 

 the tip of the bud this outer capsule is composed of only two 

 layers of cells. It is at this point that the developing manu- 

 brium later breaks through the surrounding capsule. Around 

 the sides of the manubrium, in the region of the umbrella, 

 the capsule is composed of four layers of cells, two layers of 

 entoderm formed by one continuous layer folded back upon 

 itself, an outer layer of ectoderm, and an inner layer of ecto- 

 derm which has been derived from the "glockenkern" cell 

 mass. 



The cavity of the manubrium and the cavity of the um- 

 brella are both continuations of the central cavity of the 

 bud, which, in turn, communicates with the cavity of the 

 peduncle. A cross-section of a bud in this stage, taken 

 through the manubrium and the umbrella (figure 8), reveals 

 the fact that the cavity of the umbrella is not uniform, but 

 shows clearly the formation of the four radial canals. The 

 germ gland rounds out to form four ridges down the sides of 



3 — Univ. Sci. Bull , Vol. IX, No. 18. 



