ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
283 
Platyhelminthes. 
Maturation and Fertilisation in Cerebratulus.* — Mr. W. R. Coe 
lias studied these phenomena in the ova of Cerebratulus marginatus 
Renier, and has found that their general course is closely similar to that 
in echinoderms, annelids, molluscs, &c. Some of the most noteworthy 
features may be cited. Only a minute portion of the chromatin network 
of the germinal vesicle enters into the formation of the sixteen ring- 
shaped chromosomes found in the first polar spindle ; the rest is absorbed 
by the cytoplasm. The aster-fibres at the poles of the second polar 
spindle are in part formed de novo, and are in part identical with those 
at the inner pole of the first polar spindle which have been transferred 
to the new centres. The centrosomes remaining after maturation become 
lost among the chromosomal vesicles which fuse to form the female pro- 
nucleus. 
The centrosome of the sperm-aster early divides into two ; the 
daughter-centrosomes move apart with the formation of a beautiful cen- 
tral spindle. The fibres of the original aster become arranged at right 
angles to this central spindle ; those at its ends are collected into two 
groups about the daughter centrosomes, while those nearer its equator 
degenerate. The central spindle is torn asunder by the further separa- 
tion of the centrosomes, and its fibres are apparently transformed into 
aster-fibres. The two asters thus formed may separate widely or may 
remain near together. They may slightly precede the sperm-nucleus 
towards the centre of the egg, or may leave it far behind. In all cases, 
however, they come to lie near together between or beside the germ- 
nuclei when these are nearly ready for fusion. The sperm-asters reach 
their greatest development just before the union of the germ-nuclei ; 
thereafter they begin to degenerate. The cleavage-asters have absolutely 
no relation with the sperm-asters, unless it be in the possession of the 
same centrosomes. The centrosomes are always very small, though 
sharply defined ; their centrospheres are at first homogeneous, but in- 
crease enormously in size, and acquire a distinct vesicular structure as 
in echinoderms ; about each centrosome an exceedingly delicate aster is 
formed out of the reticulum of the centrosphere. The plane of cleavage 
passes through the region where the polar bodies were formed. One of 
the most remarkable features is the great persistence of the degenerating 
aster-fibres in the inner aster of the second polar spindle, in the sperm- 
asters, and in the cleavage-asters. 
Central Nervous System of Cestoda.f — Dr. Ludwig Cohn has studied 
this in eight species of Taenia , in Boihriocephalus rugosus and B. Mans, 
in SolenopJiorus megalocephalus, Ligula digramma , and ScMstocephalus 
dimorphus. He has come to the conclusion that all the longitudinal 
strands in the scolex and in the chain of proglottides, along with all the 
commissures which connect these, constitute the central nervous system, 
as contrasted with the peripheral branches to the various organs. 
The chief commissure is only a bridge between the two main strands ; 
it forms no central organ, but merely a chiasma ; the median (dorsal and 
ventral nerves) do not spring from it, but simply communicate through 
* Zool. Jahrb. (Abth. Anat.), xii. (1899) pp. 425-76 (3 pis.). 
f Op. cit., xii. (1899) pp. 89-160 (4 pis. and 9 tigs.). 
