254 M. C. Mereschkowsky on the Hydrovda. 
quently the nutritive material attains its maximum abun- 
dance; for here the current of the radial canal unites with the 
current of the circular canal. The margin of the Medusa is 
furnished with eight lithocysts and a great quantity (more 
than thirty) of short tentacles, which are only sixteen in 
number at the moment of liberation. The manubrium is 
short, changes much in form, and is furnished at its orifice 
with four rounded lobes, The size is very variable, but it is 
usually about 6 millims. in diameter. It is completely colour- 
less, whitish; the sporosacs are slightly yellowish. By 
leaving in a marine aquarium a branch of Obelia flabellata 
with gonothece, one can always obtain as many Meduse as 
one wants. Fig. 7 a shows a Medusa of the natural size. 
The ova are large, of irregular form (Pl. XIII. figs. 8, 9, 10), 
with avery thin membrane and granular contents. In the 
middle, or more frequently near the margin, we always ob- 
serve very distinctly a large, clear and non-granular nucleus, 
more regular than the ovum itself. In the nucleus we always 
observe one or several nucleoli, and in each nucleolus a nu- 
cleolulus. All these formations are distinguished from each 
other by their behaviour with transmitted light; when the 
first of them is lighter, the second is darker, and the third 
again lighter. On changing a little the focal distance of the 
microscope all is changed ; what was dark becomes lighter, 
and vice versd. In the youngest ova we see only one nucleo- 
lus and one nucleolulus (fig. 8); the latter is usually very 
variable in its form, which is most frequently irregular. 
Sometimes it is very large (fig. 11, representing a nucleus 
very much enlarged). Further, we see ova in which the 
nucleolus has acquired a biscuit-shape, in each half of which 
we observe a nucleolulus which has evidently divided into 
two (figs. 9,14). A subsequent stage may be seen in fig. 10, 
in which the nucleolus is completely divided and each half 
contains a nusleolulus, A still more advanced stage shows 
(fig. 12) a nucleus with four nucleoli, each containing a 
nucleolulus, which is very large and variable in form. The 
form changed before my eyes with considerable rapidity, and 
the whole moved like a little Ameba. Lastly, the succeeding 
stage that I have been able to observe (fig. 13) is furnished 
with a nucleus with a great quantity (about twenty) of 
nucleoli, almost every one of which contained a very small 
nucleolulus, which, however, it was sometimes impossible to 
define. 
It is evident that all these nucleoli have originated from a 
single one by division, and that this division was always 
preceded by the division of the nucleolulus into two. Only 
