Opalina. 301 
In the latter case the “purpose” of the extrusion is positive, to 
aid in nutrition: in the former it is more negative, to free the nuclei 
from that which must in some way be a hinderance to reproduction. 
Before comparisons can confidently be made between Metazoa 
and Protozoa as to the phenomena preceeding fertilization, the former 
must be carefully studied in the light of recent work upon the 
freeing of the generative nuclei in Protozoa from nutritive chromatin 
before sexual union, and the Protozoa must be studied more success- 
fully with reference to reduction division. Bovert (1887 a, 1892, 1899) 
has shown the extrusion from the nucleus and subsequent degeneration 
of a definite part of each chromosome in those blastomeres of Ascaris 
which are to develop into soma cells, but we have no satisfactory 
understanding of the formation of vegetative chromidia in the germ 
cells of Metazoa. Yolk nuclei are well known. They seem to be 
somewhat comparable to vegetative chromidia, but their formation 
seems generally to havea positive value comparable to the formation 
of zymogen granules in metazoan gland cells, and of the chromatin 
spherules in Opalina, all bring connected with the manufacture of 
nutritive substances, in the cytoplasm. On the other hand, the 
degenerating residual chromatin in the germinal vesicles of many 
Metazoa seems more strictly comparable to vegetative chromidia. 
There is in Protozoa a somewhat similar formation of vegetative 
chromidia which degenerate before sexual union (chromatin spheres 
of Opalina, vegetative chromidia in many Plasmodroma), the result 
of the process being to free the generative nuclei from the nutritive 
chromatin which seems in some way to be an obstruction to the 
sexual process. 
Reduction. 
In the Metazoa the ripening divisions in ovogenesis and sper- 
matogenesis are accompanied by a reduction of the number of the 
chromosomes. Phenomena of this sort have been described in maturing 
Protozoan cells; by Scuaupryn (1904) and v. Prowazexk (1905) for 
Trypanosomes and by Pranpru (1905 and 1906) for Didiniwm. The 
phenomena in Paramaecium are not quite clear, but CauKins & CuLn 
(1907) interpret them as involving reduction. The accuracy of the 
observations of ScHauprnn and v. ProwazEex on Trypanosomes has 
recently been questioned by Satvin-Moore & Brerni (1907), though 
it is difficult to see upon what grounds, since they did not study 
the species or stages in which the phenomena were described as 
occuring. Dr. M. Harrmann tells me that Scuaupmyn’s further work, 
