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rOPULAR SCIENCE REVIEW. 
Synura. The differences in the results just referred to arise 
mainly from corresponding differences in the direction of the 
cleavage in the successive fissions. In Volvox , we have the first 
cleavage into two protoplasms, taking place vertically to the 
surface of the parent sphere. The second also takes place 
vertically, but in a direction at right angles to the first. The 
next division appears to take place along a plane 'parallel to 
the surface of the parent sphere. Hence, as soon as we obtain 
eight distinct protoplasms, we find them arranged in two 
parallel layers, tie thin space between the two representing 
what is ultimately destined to become the hollow interior of 
the young sphere. In Sphcerosira , on the other hand, though 
the fissions change their direction, they result in the formation 
of a disk, and not in a double series of protoplasms, because 
they are all made vertically to the surface ; the horizontal 
ones of the ordinary Volvox not taking place. When Mr. Carter 
pursued his observations on the subject, he thought he had 
found examples in which each disk consisted of 128 ciliated 
segments. If this is true, and I see no reason for doubting it, 
segmentation in his case had been repeated seven times, whilst 
in mine they were limited to five. In both instances the pro- 
cess was completed, since, in both, the separate protoplasms 
became provided with cilia — a stage of growth which, so far as 
my experience enables me to judge, always indicates the com- 
pletion of the segmentative process, and the final determination 
of the number of gonidia into which the mass is destined to be 
divided. The young organism now assumes a new character. 
Hitherto it has been passing through a stage of still life ; it is 
now furnished with locomotive organs. If we examine care- 
fully one of the disks in this completed stage, we shall find that 
it exhibits appearances represented in figures 6 and 7. In 
the former, we have a flat disk consisting of thirty-two green 
protoplasms, looking somewhat like some forms of the genus 
Pediasti'um, Fringing this disk, we can trace a number of 
freely moving cilia (6, e). Each protoplasm, in this respect, 
resembles the parent gonidium (fig. 2) from which it originated ; 
but as we watch, we observe that the disk rotates slowly ; in 
one instance I was able distinctly to trace the walls of the 
mother-cell (7, d) within which it was imprisoned. In this 
movement of the disk within the interior of the mother-cell 
we have another feature of affinity with Volvox — in which 
latter, a« I long ago pointed out, a similar movement of the 
young imprisoned sphere may occasionally be observed. In 
the Sphcerosira this revolving motion affords us facilities for 
observing the lateral aspect of the compound disk, and of the 
individual protoplasms composing it. We now see that each 
of the latter is elongated, and slightly curved. The centre of 
