66 MR, W. K. ERODES ON LUCIFER: 



spherule (?) 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 (d), 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 tune 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 rusting stage, as it had been so 



