No. 2.] ANURIDA MARITIMA. 229 
Maturation. 
As has already been seen, the germinal vesicle disappears 
early in development, before any yolk has been formed. The 
nuclear wall is lost and the chromosomes are too small for 
recognition in the greatly increased mass of cytoplasm. From 
this time onward no sign can be found of the nucleus. The 
most careful search fails to reveal anything even in the mature 
ovum just before laying. No polar body spindle has been found, 
and the first reorganization of the nucleus making it visible is 
found in the section of the egg shown in PI. XXI, Fig. 25, where 
the polar bodies have just been given off and the female pro- 
nucleus is returning to the centre. This egg has been laid 
some little time, and the male pronucleus is already at the 
centre awaiting the return of the egg nucleus. 
Naturally Anurida is a most unfavorable form for the study of 
nuclear changes. Nothing determinate has been observed. 
Gradual shrinkage is the description best fitting the only 
change observable. The changes taking place in the ova, 
shown at a.o. in Figs. 10, 11, and 12, are curious, and perhaps 
show what occur in the others. Up to a certain stage it is 
impossible to tell whether a cell will develop to an egg or one 
of these abortive ova. Fig. 9, @., may be either. There is the 
same reduction in the amount of chromatin, but instead of ulti- 
mately losing the nuclear membrane this swells up, enlarges, 
and forms a large germinal vesicle almost obliterating the cell 
body (Figs. 10, 13, @.0.). The chromosomes enlarge slightly, 
and, moving from the periphery, definitely arrange themselves 
in groups of four. This occurs at about the time of maximum 
yolk development, degeneration and absorption following soon 
after. A possible explanation of their existence may be that, 
although a certain number of germinal cells assume the charac- 
ters distinctive of ova, still there is not nutriment enough to 
mature them all; hence, some are brought to a standstill in 
their onward progress by lack of food and undergo the degen- 
erative hypertrophy spoken of before. The changes occurring 
in the nuclei of these cells may then be, in a measure, indica- 
tive of those occurring in the much smaller germinal vesicles, 
