492 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
Gemmules of Spongillidae.* — Dr. W. Weltner, after some account 
of the structure and history of the gemmules of fresh-water Sponges, 
points out that our knowledge of their developmental history is still 
incomplete ; there are two primary questions which still need to be 
answered — the origin and nature of the cells which form the foundations 
of the gemmule, and the fate of these cells. It is possible that the 
gemmule is formed from a single cell which has the value of an ovum, 
in which case the gemmule would have to be regarded as a mass of 
segmenting cells. But the internal mass of the gemmule may arise 
from several equal cells of the mesoderm, or from several unequal cells, 
or, finally, the gemmule may be formed by cells of two or three germinal 
layers, in which case the gemmule would be a bud. 
Protozoa. 
Stigmata of Mastigophora.f — Herr E. Franze has made a study of 
the stigmata or eye-spots of Flagellate Infusoria. He finds them to be 
the simplest optic organs, and to consist of a plasmatic finely marked 
ground substance ; in this there are deposited numerous oily red 
granules, and one or a few or many highly refractive granules ; these 
latter, in the Euglenoidea, consist of paramylum, and in other Masti- 
gophora of amylum ; the granules are generally regularly arranged, 
but are sometimes irregular, and they exhibit a differentiation into larger, 
central, or excentric crystalline bodies, and smaller, always more 
numerous, lens-bodies. The larger granules are generally imbedded in 
the pigment, while the smaller bodies lie on it. 
Besides these stigmata there are others which consist of one layer 
of grains of amylum, and a layer of pigment which completely surrounds 
it. Stigmata of this kind are, as a rule, found in Chlamydomonads, 
Volvocinea, Dinobryinea, and a few others. 
The stigmata serve as organs for the perception of light ; the crystal- 
line body concentrates the light, the lens-bodies serve to concentrate 
the perception of light, and the pigment-layer is not only light- 
absorbing, but also light-perceiving. They are also capable of perceiv- 
ing heat, and the influence of light produces thermotactic movements, 
which are either thermophilous or thermophobic. 
The eyes of Turbellarians and Eotifers are not homologues of the 
stigmata ; the external resemblance between the two kinds of differentia- 
tions are due to similarity of function. The so-called oral-ridge 
( Mundleiste ) of various Monads is probably composed of highly re- 
fractive granules ; what their true function is is still uncertain, but it is 
certain that they are not specific light-perceiving organs. 
Merotomy of Ciliated Infusoria.^ — Prof. E. G. Balbiani finds that 
in the Ciliated Infusoria, which may be considered as one of the most 
favourable types for the physiological study of cells, certain functions 
are fulfilled by the protoplasm alone, and others by it and the nucleus. 
The former are the different modes of movement, and the power of 
directing the body during progression. The latter are the various 
* Biol. Centralbl., xiii. (1893) pp. 119-26. 
f Zeitschr. f wiss. Zool., lvi. (1893) pp. 138-64 (1 pi.). 
% Ann. de Micrograpliie, v. (1893) pp. 1-25, 49-84, 113-37 (2 pis.). 
