THE MEDUSiE OF MILLEPOEA MUKEAYI. 379 



separately in the ectoderm of the zooids, but in the majority 

 of cases there is but a single cluster or aggregation (figs. 2, 

 3, and 4). The largest and most fully developed of these lie 

 at the apex of the zooids (figs. 5, 6, and 7), 



The conclusions from these facts seem to be that the germ- 

 cells developing in the canals until they reach the stage corre- 

 sponding to the sperm-morula or spermosphere migrate towards 

 the zooids, fusing into aggregations as they do so. Having 

 reached the zooids they take up a position between the ecto- 

 derm and endoderm' of their apices, and continue there the 

 later stages of their development. 



The spermospheres are most frequently found in the dacty- 

 lozooids, but in a few cases I have found them in gastro- 

 zooids (fig. 3). They have probably no preference for 

 either the one form or the other ; but they are found 

 more frequently in the dactylozooids, partly because these 

 forms are more numerous, and partly because the gas- 

 trozooids are usually more remote from the larger coenosar- 

 cal canals. 



The spermarium having been formed at the apex of the 

 zooid certain noticeable changes take place. In the first place 

 by a thickening of the ectoderm the pore becomes narrowed 

 (figs. 5, 6, and 7). The tentacles become flattened out, and 

 the neraatocysts disappear. The spermarium sinks into a cup- 

 shaped receptacle on the summit of the zooid, and the endo- 

 derm of the edge of the cup grows out, pushing before it the 

 ectoderm. 



These changes are represented in the two figs. 6 and 7. In 

 the next stage the cup-shaped receptacle of the spermarium 

 has grown out into a bell-shaped structure (fig. 8). The sper- 

 marium is much larger in size, and the pore is completely 

 closed by ectoderm. In the later stages (figs. 9, 10, and 11) 

 the following changes may be noted. The operculum is formed, 

 shutting off all access from the cavity of the gonangium to the 

 exterior. The walls of the bell-shaped outgrowth become 

 considerably attenuated, and lie close against the ectodermic 

 wall of the ampulla. The manubrium is formed probably by 



