250 BULLETIN OF THE 
in the same plane as in former instances, the line indicating the first 
plane of cleavage, which in them was unbroken, is bent at right angles 
at the point where the secondary furrow has appeared. The second 
cleavage furrow is at this time a little over .15mm. long. The longer 
diameter of the egg is .60 mm.; the shorter, about .45 mm. 
The general appearance of the egg two minutes later than the last, or 
at 1h. 20m. p.. (fig. 17), although in most respects similar to it, has 
several marked differences, the result of the progressive growth. One 
of the most striking of these differences is the still greater increase in 
the amount of the deviation from a straight line which now separates 
the lower end of the upper line from the upper end of the lower ver- 
tical, both being the profile of the first cleavage plane, 1 cl. pl. The 
length of the second cleavage furrow, se., has now increased to .25 mm., 
its breadth remaining about the same, and in its sides are frequent pli- 
cations running parallel with the first cleavage plane, much more 
sharply defined than in any which has preceded it. Up to the present 
time (fig. 17), twenty minutes after the first visible changes by which 
we pass from an egg with two cells into one with four, the secondary 
furrow has been limited in its extension. It now slowly deepens, and 
at the same time grows along the surface of the ovum toward the 
equator, although at 1h. 20m. it has not yet extended far enough to 
reach the periphery of the egg as seen in profile. There is as yet no 
indentation marking the limit of the second cleavage groove on the 
equator of the egg. 
In the same egg five minutes later, at 1h. 25m. p.m. (fig. 18), the 
second furrow, se., is found extending across the whole hemisphere, and 
is represented in the figure by the large horizontally placed furrow. 
The size and depth of this indentation may be estimated by the depres- 
sion at either extremity of this furrow. In profile it is seen to equal in 
depth the radius of the egg. Like the primary groove, pr., this likewise 
eventually extends almost through the egg, dividing it into two sym- 
metrical hemispheres connected by an undivided “bridge.” The walls 
of the furrow, still grooved with cleavage folds, have not yet begun to 
approximate. In this stage (fig. 18), although we seem to have four 
segmentation spheres, the second plane of cleavage does not extend 
more than two thirds across the diameter of the egg as seen from the 
original surface of infolding. On the side of the egg away from the 
observer, the 2-cell stage was slightly grooved by the second furrow. 
The sides of the second cleavage furrow have not yet begun to draw 
together. At this time in the growth of the ovum the walls of the 
