STUDIES ON THE COMPARATIVE ANATOMY OP SPONGES. 349 
outer layer of columnar epithelium and a few “ rosette-cells ” 
in the interior. The origin of these so-called “ rosette-cells ” 
is unknown; from the figures, copied by Yosmaer, they appear 
to consist each of a spherical ball composed of a few small cells 
arranged around a small central cavity. The rosette-cells 
increase until finally they fill the whole cavity of the embryo. 
Metschnikoff regards the rosette-cells as mesoblastic structures, 
aud he states also that amoeboid cells wander in amongst them 
from the epiblast ; the canals commence as isolated spaces in 
in the mesoblast. Vosmaer gives no further account, but from 
what he says, aud from the figures, it seems very probable that 
the so-called rosette-cells are the young flagellated chambers, 
and that they have themselves been formed each from a single 
large amoeboid cell derived from the epiblast. 
Goette (10) gives a long account of the development of 
Spongilla, and Yosmaer tells us in his abstract that “Die 
Geisselkammern entwickeln sich getrennt von einander und 
von anderen Hohlraumen ‘ vermittelst einer Knospenbildung 
einzelner Zellen.’ M 
Development of Stelospongus flabelliformis. 
(a) The Ovum. 
The ovum appears in section as a somewhat oval cell lying in 
a fibrous capsule as described above. The body of the ovum 
is granular and deeply staining. At one pole there is a large 
oval nucleus with a very definite wall, and right up against 
the wall, at the outer pole of the nucleus, there lies a small, 
spherical nucleolus. The nucleus does not contain nearly so 
many granules in proportion to its size as the body of the 
ovum, but there is a quantity of coarsely granular material 
chiefly aggregated towards the pole, remote from the nucleolus. 
The nucleolus stains deeply, and is almost, if not quite, homo- 
geneous. The longer diameter of the ovum measures 0-076 mm., 
of the nucleus 0 024 mm., and of the nucleolus 0 0048 mm. 
