MARCH: GERM CELLS OF CORYMORPHA. 253 



the arrangement of the chromatin, gives to the nucleus a 

 hollow, rounded appearance which is absolutely distinctive and 

 serves as the safest means of identification. The cytoplasm 

 of the primitive germ cell is usually more dense than that of 

 other cells, and contains coarse, deeply staining granules, 

 probably mitochondria. The limits of the primitive germ 

 cells are usually quite definite, marking off the germ cells 

 from other cells among which they are found. 



2. — Origin of the germ cells. Primitive germ cells appear 

 scattered throughout all parts of the ectoderm of the peduncle, 

 but are most numerous near the bases of older medusae in 

 regions where new buds are developing (figure 1). 



3. — Migration of the germ cells. In order to follow the path 

 of the migrating germ cells it has been necessary to arbitrarily 

 establish certain stages in the development of the medusa bud. 

 These stages are based entirely upon differences in structure. 

 Since all of the material used for these observations was fixed 

 material, there is absolutely no way of measuring the length 

 of time involved in the development of the successive stages. 



Stage A includes all buds from the merest outpushing up to 

 stage B (figure 2). 



Stage B includes only the elongated buds with a distinct 

 thickening of the ectoderm at the apex (figure 3) . 



Stage C includes those buds having the undifferentiated 

 "glockenkern" cell mass between the ectoderm and entoderm 

 at the tip of the bud (figure 4) . 



Stage D includes all buds having the pear-shaped "glocken- 

 kern" (figure 5). 



Stage E includes buds similar to the one shown in figure 6. 



Primitive germ cells are found scattered throughout the 

 ectoderm of the peduncle, but are not seen among the ordinary 

 entoderm cells. In all cases where there is any indication of 

 the formation of a medusa bud, and in all later stages, germ 

 cells are found among the ectoderm cells of the young bud. 

 Whether the presence of germ cells serves as a stimulus for 

 the development of the bud, or whether the development of 

 the bud attracts the germ cells, can not be determined, but the 

 presence of germ cells in the entoderm of the earliest buds can 

 not be denied. The fact that germ cells are normally present 

 in the ectoderm, and are not present in the entoderm, except 



