280 R. T. GUNTHER. 
give rise to the subumbrella side of the velum—the outer side 
of the velum being formed from the second ingrowth of ecto- 
derm, as will appear in the sequel. 
By the next stage the forecasts of most of the important 
organs of the Medusa can be readily recognised (figs. 13 and 
13a). The subumbrella ectoderm (s. ect.) and velar ecto- 
derm (v. ect.) have become much thinner relatively. The sub- 
umbrella cavity (s.c.) is very large. The endoderm cells 
surrounding the “ glockenkern” have grown further over that 
organ, and at certain regions have begun to grow up into the 
mass of ectodermic cells at the apex of the bud, thus indicating 
the positions of the future tentacles. At this early stage the 
forecasts of the tentacles are solid, but in section their endo- 
dermal axis (¢. end.) may be shown to be composed of nume- 
rous cells arranged round a central axis. A lumen does not 
appear until a somewhat later stage. Of the tentacles, four 
are greatly in advance of the others, but I was not able to 
make out whether two of these make their appearance before 
the other two, as is often the case among the Hydrozoa. 
Meanwhile a great change takes place in the apical mass of 
ectoderm. Its constituent cell nuclei, instead of being more 
or less uniformly diffused throughout the mass, now assume 
definite positions with regard to the neighbouring organs 
(fig. 18). The more deeply seated nuclei range themselves in 
a single tier along the outer side of the “ glockenkern,” while 
the others surround the forecasts of the endodermal axes of 
the tentacles. In the section through a Medusa bud of this 
stage in fig. 13, a small space may be seen between the 
ectodermic epithelium of the tentacles and the ectoderm over- 
lying the ‘“glockenkern.” The subumbrella cavity at this 
stage still remains completely closed by a membrane composed 
of part of the “ glockenkern” and of the overlying layer of 
ectoderm. This membrane is the forecast of the velum and 
subsequently will become perforated. 
In the most advanced stage observed (fig. 14), the velum no 
longer extends right across the mouth of the subumbrella 
cavity, but has broken through in the middle. The tentacles 
