294 THE AMERICAN MONTHLY [Sept. 



hydroid. The endoderm (d, e) has now grown out around 

 the edge of the bud, forming a deep cup, and has also 

 made an evagination (b) which is the beginning of the 

 proboscis. The two endodermal Layers (d, e) forming 

 the cup, remain, for a time, entirely separate. Subse- 

 quently these two layers grow together with the excep- 

 tion, first, of the large four-cornered cavity (c) which be- 

 comes the stomach, secondly, of the four radial canals 

 (fig. 10, a), and thirdly the circular canal (fig. 9, c) which 

 is connected with the stomach by the radial canals. 

 The bud now changes from a nearly spherical shape to a 

 broadly discoid form (fig. 8) and here seems to be the be- 

 ginning of an important step, which is the gradual 

 broadening of the developing bud to form the Campanu- 

 larian medusa, instead of retaining its spherical form and 

 developing into the Tubularian medusa. The proboscis 

 (b) has now become much more prominent; while at the 

 same time, the two ectodermal layers (c, d) have become 

 thinner over the proboscis and subsequently break 

 through, forming the opening to what corresponds to the 

 bell-cavity of the Tubularian Medusa, or the sub-umbrella 

 cavity. We now have the sub-umbrella cavity lined with 

 a layer of cells of ectodermal origin. This layer unites with 

 the ectoderm of the outside of the bud, thereby forming the 

 edge of the disk which surrounds the sub-umbrella 

 cavity. We thus have one continuous layer of ectod- 

 ermal cells covering the outside of the bud and lining the 

 sub-umbrella cavity. The tentacles make their appear- 

 ance as buds (fig. 9, b) on the edge of the disk. These 

 buds are outgrowths of both ectoderm and endoderm, so 

 that the tentacles contain both the ectodermal and endo- 

 dermal cell layers. As the tentacles grow they curl in- 

 wardly upon themselves, so that, until the time of birth, 

 they appear as broad crenulations (fig. 2, a). The mouth 

 also makes its appearance by virtue of a separation of 

 the cells (a) at the end of the proboscis. 



