42 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
In the deepest portions of the ovary the spherical nuclei of the egg- 
cells are regularly provided with a centrally-placed nucleolus, which 
possesses the characters of a plasmosoma. Karyokinetic figures appear 
in some numbers notwithstanding the small size of the cell. Within the 
ovaries the egg-cells have a delicate plexiform structure, which is barely 
noticeable, and paraplasmatic contents are not to be seen. As they pass 
to the oviducts the spherical form is more and more replaced by the 
pyramidal, and the dimensions of all the parts increase, though not 
regularly. As maturation proceeds, yolk-spherules appear in the body 
of the egg-cell and lie in the rounded meshes of the protoplasmic net- 
work ; the nuclei of these cells become stellate, and no distinct mem- 
brane can be demonstrated ; the nuclear substance appears to be almost 
homogeneous, and karyokinetic figures are only rarely seen. 
The structure of the nuclei of the egg-cells does not long remain 
simple ; very many of the stellate nuclei give off new elements, which 
are of great importance in the formation of the chromatin elements 
which pass into the polar corpuscles. 
In the parts of the oviduct which lie nearer the uterus the egg-cells 
take on a more or less spherical form, and further changes go on in their 
nuclei. The formation of polar bodies is cotemporaneous with the entry 
of the spermatozoa into the egg-cell ; ordinarily only one sperm-cell 
enters the egg-cell ; it then soon undergoes a peculiar disintegration ; 
the head becomes rounded, and instead of having the form of a horn, is 
more or less spherical ; it separates from the other parts of the sperma- 
tozoon. It now either lies freely in the body of the egg-cell, or is 
surrounded by a small quantity of a peculiar substance which appears 
to have the characters of protoplasm. 
The male and female pronuclei appear to be exactly similar, save 
that one is ordinarily larger than the other ; their karyokinetic meta- 
morphoses exhibit some remarkable peculiarities, the loops becoming 
well stained with safranin, which is not the case when the polar bodies 
are being formed, and each loop is made up of granules arranged in the 
fashion of a rosary. The blastomeres have nuclei which are similar to 
the pronuclei. 
The author concludes by pointing out the great value of the method 
of combined staining in distinguishing the various processes which go 
on in developing and fertilized ova. 
7. Platyhelminthes. 
Development of Distomum macrostomum.* — Dr. G. A. Heckert 
gives a monographic account of LeucocMoridium paradoxum and its adult 
form Distomim macrostomum. The larvm have long been known as 
brightly coloured vesicles in the horns of the snail Saccinea amphibia, 
but Zeller was the first to demonstrate (in 1874) the connection between 
these and the adult Distomum parasitic in singing birds. What Zeller 
did in outline, Heckert has completed in detail. He starts from the 
sporocyst threads which penetrate the liver of the snail in all directions, 
and describes how parts of this meshwork acquire with the growth of the 
germs a very different structure, becoming brightly coloured pulsating 
♦ Bibliotheca Zoologica (Lcuckart and Chun), Heft iv. (1889) pp. G6 (4 pis.). 
