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MR. w. It. BROOKS OK LlTClFBR: 
spherule (c) is completely divided by a radial fissure into two, and these project into 
the segmentation cavity (b) a little more than they did before. 
In fig. 15 the flattening has become a deep pit (cl), and the spherules (c) have been 
pushed quite into the segmentation cavity, and the adjacent cells have begun to move 
in the same direction. This change is more marked in fig. 16 ; and in fig. 17 the egg 
consists of a double wall of cells, the ectoderm and the endoderm, surrounding a 
primitive digestive cavity (d), and separated from each other by the segmentation 
cavity ( b ), in which the two cells (c) are situated. Each of these also shows traces of 
a division into two. 
These changes are more marked in fig. 19 ; and in fig. 20 the opening of the 
primitive digestive cavity is much reduced in size, and the cavity itself does not lie 
exactly in the axis of the egg, but at one side of it. 
A more minute examination of the segmentation brings out a number of interesting 
points ; one of them is the rhythmical character of the process, which is not a con¬ 
tinuous uniform change, but a series of stages of activity, separated from each other by 
periods of rest. 
The egg shown in Plate 1, fig. 1, was laid about 10 o’clock p.m., and about 10.35 it 
was in the condition which is represented in the figure. As I had not been watching 
it I did not observe the first division, and when first seen it was in the resting 
condition, and the two spherules were not sharply defined, but pressed together. 
During the next fifteen minutes no external change was visible, and the drawing 
was made at 10.50 p.m. It then entered upon the second period of segmenting activity, 
and in five minutes the two spherules were well defined, as shown in fig. 2; and in 
five minutes more (fig. 3) one of them showed traces of division into two. In ten 
minutes more (fig. 4) this division was completed, and traces of a similar change had 
made their appearance in the other spherule, which was also perfectly divided into 
two at the end of five minutes more (fig. 5). This stage ended the second period 
of activity, which was twenty-five minutes long. 
During the whole of this time the egg showed gradual and uniform change, which 
was sufficiently rapid to be distinctly visible. Although four so-called stages are 
figured, there was no division into stages, but a continuous change without interruption. 
The four spherules now began to flatten down, and in five minutes the egg was in 
the condition which is shown in fig. 6, and it then remained without any external 
change for more than ten minutes. The second period of rest, measured from the time 
when the four spherules began to shrink together to the time when they began to 
swell out and enter upon the third period of active segmentation, was therefore more 
than fifteen minutes long. 
At 11.40 the four spherules were once more sharply defined (fig. 7), and changes 
went on uniformly until, at 12.15 A.M., each was perfectly divided into two, as shown 
in fig. 8, which marks the end of the third period of activity, thirty-five minutes long. 
I was not able to watch this egg pass into the next resting stage, as it had been so 
