252 BULLETIN OF THE 
At 1h. 45m. p.™. (fig. 21) the protoplasmic forces are still active in 
sending out the rhizopodia, and the secondary cleavage plane (2 c/. pl.) 
is not wholly formed, and at 1h.55m. p.m. (fig. 22) the 4-cell stage 
is practically complete, although here and there, as at the left of the 
figure, a slight protoplasmic elevation van be seen. The second plane 
of cleavage is practically formed. 
An hour and ten minutes, 2h. 10m. P.M. (fig. 23), after the 2-cell 
stage we have an egg divided into four cells by two planes at right 
angles to each other. None of these cells have a nucleus, and all are 
still penetrated by the network of ‘‘cells” which we have already de- 
scribed in the unsegmented ovum. The vertical plane passes through 
a rosy region of the egg; the opposite pole is more transparent. The 
diameter of the egg on the first cleavage plane is a little less than 
.50 mm.; on the opposite plane, about .45 mm. Although on the face 
. of the egg which is before us the ovum is divided into the 4-cell con- 
dition, I have not been able to observe the opposite pole. Subsequent 
stages seem to indicate that the secondary plane does not extend 
wholly through it, but that at the opposite side there still remains 
an undivided surface. Later changes in the general outlines of the 
ovum lead me to suspect that the undivided part, either by growth 
or protoplasmic extension, is of considerable size after the formation of 
the 4-cell stage. 
Third Cleavage Furrow.— The appearance of another cleavage furrow 
on the same egg, the third which has been traced, was first noticed at 
3h. 15m. P.M. (Pl. II. fig. 3), two hours and thirty minutes after the 
2-cell stage. In the mean time certain changes in the contour of the 
egg which are not fully understood had taken place. At 3h. p.m. (PI. 
Th, fig. 1) the primary (lc/. pl.) and secondary (2 cl. pl.) planes of 
cleavage, represented by the vertical and horizontal planes, occupy the 
same relative position as formerly, and the right-hand cells are in the 
main the same in contour. On the side of the left-hand cells, as fig- 
ured, away from the observer, has appeared a large undivided lobe (et.), 
a little smaller than the original left-hand cell of the 2-cell stage. 
The egg has probably been slightly rolled on its axis, by which the large 
undivided lobe is turned into sight, whereas formerly it was concealed 
behind the two left-hand cells of the 4-cell stage. I was not able to 
observe satisfactorily the origin of this large lobe. The only explana- 
tion which can at present be given to account for its existence is one 
suggested above, that it is the bridge or connecting band which has not 
been divided by the second cleavage furrow. If, however, its fate 
