LIFE-HISTORY OF XHNOPUS LAVIS, DAUD. 801 
eggs on the second day after spawning. In other respects the egg shows the normal 
appearance before segmentation commences. 
Segmentation.—The details of segmentation do not appear to differ much from 
those seen in small eggs of other Anura, and are, therefore, not described in detail. 
I have had figures of a few stages made as accurately as possible to show the 
general course of the processes. 
The first furrow is, as usual in Anura, meridional and divides the eggs into two 
equal blastomeres. It is completed one hour to one and a half hours after fertilisa- 
tion. ‘The next vertical furrow appears within two and a half hours after fertilisation, 
and the third (horizontal) one within the next hour. The third furrow does not form 
exactly parallel to the equator, but is bent in the manner shown in fig. 2, Plate LI. 
The egg is viewed here from the right side, according to the statement made by O. 
ScHuttze and Kopscx to the effect that the unpigmented portion of the egg reaches 
much nearer the upper pole on that side of the egg which is to become the posterior 
end of the embryo. In this egg there is at the upper pole a marked departure from the 
radial symmetry which, as shown in fig. 24, is still present at the lower pole. The egg 
has become bilaterally symmetrical with an elongated and a more rounded cell on each 
side at the upper pole. The pigment is not altogether confined to the cells of the upper 
hemisphere ; there is a patch posterior to them (fig. 2, Plate 1). The egg represented 
in figs. 3 and 3a, Plate I, is at a stage reached about four hours after fertilisation. 
The segmentation has become irregular, especially of the yolk cells. There is still 
a marked difference in the bulk of the cells of the upper and lower hemispheres, 
but the latter are now rapidly dividing. Furrows start from the edge of the 
pigmented areas and extend downwards over the yolk cells until they meet an 
existing furrow near to, but not accurately at, the lower pole of the egg. At the 
stage shown in fig. 4, the cells of the upper and lower hemispheres are almost equal 
in size and are again arranged fairly regularly. This return to a regular arrangement 
must be a result of the mobility of the superficial cells, together with the need for a 
geometrical arrangement in order to accommodate a given number of cells of a certain 
size in a limited space. That the cells of both the upper and lower poles are movable 
to a certain extent can easily be observed in the living egg. Newly divided cells 
may push apart two cells which were in contact with each other until the edges 
which were touching are separated by the whole diameter of the intruding cells. 
If there are intercellular strands of protoplasm at this stage they must certainly 
become broken in the shifting about of the cells. 
At the stage shown in fig. 5, the cells have become smaller and have lost both 
the regular arrangement and the roughly hexagonal outlines seen in the earlier 
stage. The yolk cells near the equator are dividing more rapidly than those 
at the yolk pole and are appreciably smaller. The living segmenting egg at this 
stage has a feature which is shown here in the figure (fig. 5) of a preserved specimen. 
A number of cells at the margin of the spreading epiblast are of much paler brown 
