GERM CELLS OF COELENTERATES 619 



DEVELOPMENT OF MEDUSA BUDS 



The. first indication of the new bud is an outgrowth from the 

 tentacle base, and this outgrowth includes both the ectoderm 

 and the entoderm of the tentacle. Figure 45 shows a small 

 and a larger evagination, both of which are to produce medusae. 

 In the drawings of such a scale the character of the cytoplasm 

 is not represented, nor is the nucleoplasm indicated; this is not 

 a serious omission since it may be said that all the cells of these 

 outgrowths are practically identical in appearance and character 

 of their contents. The cytoplasm is a compact, non-vacuolated, 

 rather deeply staining substance, the nuclei are all large and 

 filled uniformly with granules and contain a prominent nucleolus. 

 There is some difference in the shape and size of these cells but 

 these are the only distinctions to be observed in the evaginations. 



Figure 46 is a slightly later stage in the formation of the 

 medusa bud, the ectoderm growing inward and pushing the ento- 

 derm back before it. Figure 47 is a bud of about the same 

 stage of development, or slightly earlier, and magnified some- 

 what more, in which the ectoderm cells are increasing in num- 

 bers but have not yet grown so far inward. Some of the ento- 

 derm cells are assuming a columnar form while the ectoderm 

 cells remain polygonal. Figure 48 is an older bud in which the 

 proliferating cells of the ectoderm are growing inward on either 

 side in several sheets and pushing the entoderm cells still further 

 back. The sheets of ectodermal cells growing inward on the 

 two sides have already spl t into two layers and these are to form 

 the ectodermal lining of the sub-umbrella and the ectodermal 

 covering of the stomach in the adult medusae. The two strands 

 of entoderm which extend furthest toward the outer margin of 

 the bud are beginning to form two of the radial canals. In 

 nearly all of these stages it will be seen that the entoderm cells 

 are the first to change their form and assume something of the 

 columnar shape characteristic of such cells in mature medusae. 

 Likewise in all these stages the ectoderm cells remain polygonal 

 in form. 



