STUDIES OF FERTILIZATION 441 
polar bodies are present. This condition, as I have already said, 
is the commonest condition in such a lot of eggs, and both polar 
bodies are invariably present. 
Attention may be directed to the fact that each chromosome 
is embedded in a homogeneous ground substance of about the 
same tint, in the iron haematoxylin stain employed, as the cyto- 
plasm. Evidently, each chromosome with its surrounding ma- 
trix corresponds to a single chromosomal vesicle of the early egg 
nucleus; the numbers are the same. The chromosome of the 
succeeding cell generation arises within the substance of the chro- 
mosome of the preceding generation in this case. 
The second class of cases, monasters, is illustrated in fig. 44. 
I was at first inclined to think that these might be due to fertili- 
zation with partial sperm nuclei, especially as the degree of devel- 
opment of the monaster shows a wide range of variation. But 
careful study of the eggs concerned showed that the first polar 
body had invariably failed to form, and the second was always 
present alone. About twenty-five cases of this kind have been 
examined without a single exception occurring. 
In the second study of this series (Lillie, 11) I have described 
the cause of failure of the first polar body. This condition 
occurs in eggs centrifuged just before the formation of the 
first polar body. The first maturation division may then take 
place within the egg forming two nuclei, and the second matura- 
tion spindle which involves both nuclei is a tetraster (Lillie, ’11, 
fig. 6). The second polar body is formed from one pole of the 
tetraster and three nuclei are left in the egg, which soon unite. 
Under these circumstances, if the sperm nucleus be absent, a 
more or less feeble monaster may develop at the time of the 
first cleavage; though in a few cases where only the second polar 
body was formed, no signs of radiations were found. 
There is thus a more or less striking difference in the behavior 
of the egg nucleus in those cases where both polar bodies are 
formed and those in which the first polar body is suppressed. It 
is a very interesting problem whether the formation of the mon- 
aster in the latter case is a purely quantitative relation, due to 
the larger number of chromosomes present in such cases? There 
THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 12, No, 4 
