788 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
arrangements which need not be detailed here he seems to have succeeded 
in showing how the fishes may he provided with abundant food. 
Oogenesis in Cyclops and Canthocamptus.* — Dr. V. Hacker finds 
in the oogenesis of Canthocamptus staphylinus that the longitudinal 
splitting of chromatin elements in the coil stage has nothing to do either 
with a primary or with a secondary longitudinal splitting of the elements 
of the directive spindle, but depends on a special process of doubling 
(diplosis) which is interpolated between the last division of the primitive 
ova and the definite appearance of a germinal vesicle. In the directive 
spindles two successive reductions occur, so that the number of chromo- 
somata (doubled by diplosis) is quartered or reduced to half of that 
characteristic of the species. The divisions of the primitive ova are 
followed by a separation, splitting, and rearrangement of the chromo- 
somata, the final result being a longitudinally cleft chromatin loop. 
There is no resting state represented by a chromatin reticulum. Certain 
changes initiating the stage of maturation give rise to a simple plate-like 
or lens-like mass ; this is succeeded by a ring and a double plate, from 
which by subsequent splitting there arise four rods. Each rod is com- 
posed of six spherical segments, at the equator of which, at right angles 
to the longitudinal axis of the rods, the chromatin substance lies. Of the 
four rods, two pass into the first polar body, one into the second, and 
one remains in the female pronucleus. A longitudinal cleavage of the 
chromatin elements persists throughout. 
In the same way in Cyclops strenuus and C. signatus the longitudinal 
cleavage of the chromatin threads follows the division of the primitive 
ova, and the double system persists throughout the entire growth-period 
of the egg. Before the beginning of the polar-body formation four 
double segments appear which are divided into eight double rods. Two 
remain in the egg. Both the divisions of the maturation period occur in 
the oviduct. The shortening of the oogenesis in these cases is, perhaps, 
an adaptation to the “ semi-pelagic ” character of the species ; on the 
other hand, in perennially reproductive species, such as C. brevicornis, 
C. tenuicornis, C. agilis, and C. pulchellus, there is a typical resting stage 
of the nucleus during the growth-period, and a consequent prolongation 
of oogenesis. The frequent occurrence of a resting blastoderm stage in 
Cladocera is, perhaps, likewise adaptive. Dr. Hacker has been able to 
trace the return of the second polar body into one of the segmentation 
cells. In the stage with two segmentation cells each nucleus is still 
visibly composed of two distinct halves. 
British Cladocera.f— Mr. D. J. Scourfield records as British Cerio- 
daphnia megops G. O. Sars, C. quadrangula O. F. M., Daphnia hyalina 
Leydig, D. galeata G. O. Sars, Atona intermedia G. O. Sars, and Chydorus 
ovalis Kurz. In some of these the female alone is known. 
Vermes. 
a. Annelida. 
Sensory Epithelia of Annelid Worms. :£ — M. E. Jourdan has investi- 
gated the sensory epithelia of Bhynchobolus siphonostoma Clap., Syllis 
* Zool. Jahrb., v. (1892) pp. 211-48 (1 pi.). 
t Journ. Quek. Micr. Club, v. (1892) pp. 63-9 (2 pis.). 
% Ann. Sci. Nat., xiii. (1892) pp. 227-58 (2 pis.). 
