M. C. Mereschkowsky on the Hydroida. 253 
is easy to see that the first stage consists in the depression of 
the ectoderm (it is the latter that is active here), and that 
the first indications of radial canals are not the result of the 
expulsion of the endoderm into the ectoderm, but, on the con- 
trary, of the ¢mpulsion of the ectoderm into the endoderm. 
The latter continues quite passive ; it submits to the action 
of the ectoderm, which of itself begins to thicken, and by this 
means to bury itself in the endoderm. The following stages 
(figs. 3, 4, and 5) are only the more advanced stages of 
the process which we have already analyzed: the ectoderm 
becomes more and more developed, at the same time that it 
buries itself in the interior, leaving in their place only the 
four spots, which thus become converted into radial canals. 
In fig. 8 the apex of the cavity is already excavated, and there 
are faint indications of the four canals (only two are repre- 
sented); but speedily this cavity again becomes convex, and 
it is from this moment only that the endoderm becomes active; 
it thickens in the middle (fig. 4), and begins in its turn to 
penetrate the ectoderm in order to form the manubrium. In 
fig. 5 we see the same stage with the four canals. It will be 
seen that between the two neighbouring canals there is only a 
uniform layer of ectoderm in which they are immersed, which 
proves that F’, Schultze is right in not admitting any interme- 
diate layer between the canals and distinct from the ectoderm, 
as was done by Agassiz. After all this, according to F. 
Schultze, the ectoderm itself splits into two layers, one of 
which produces the muscular sac belonging to the umbrella, 
and the other forms the superior layer of the manubrium*. 
Unfortunately various circumstances drew me away from 
these observations ; so that I have not seen the stage inter- 
mediate between fig. 5 and fig. 6, in which the Medusa is 
ready to detach itself. 
As the Medusa of Obelia flabellata is, so far as I know, 
undescribed, I will give a short account ofit. Pl. XIII. fig. 7 
represents a mature example, and shows that this Medusa 
differs very little from other Meduse of the same genus, as, 
for example, that of Obelia dichotoma figured by Hincks fF. 
The umbrella is very flat, but slightly campanulate, without 
thread-cells, with four radial canals, four oval sporosacs filled 
with ova and placed at the extremities of the four canals, 
where they unite with the circular canal, and where conse- 
* By this the development of the Medusa of Obelia flabellata differs 
from that of Corymorpha nutans, in which the division of the ectoderm 
takes place sooner, as early as the first indications of the radial canals 
(Allman, Monogr. Gymnobl. Hydr. p. 77). 
+ Loc. cit. pl. xxviii. fig. 1, ¢, d. 
