166 WALTER HEAPE. 
Four segments now make their appearance by the division 
of the first two (fig. 12). Each of the segments is of different 
size, and indeed in every ovum which I have examined of this 
stage with one exception, such is the case. (For measure- 
ments vide table p. 169.) 
Spermatozoa and polar bodies are still to be seen in the 
circum-vitelline space and have been found in ova as old as 
fifteen segments, although the former in fewer numbers and 
both considerably shrunk. 
From this point the segmentation continues entirely 
irregularly, and the segments formed are of various sizes. 
Figs. 18 to 19 are sketches of ova with six, seven, eight, 
nine, seventeen, and larger numbers of segments. A table of 
the measurements of the segments of several of them will be 
found on p. 169. 
Throughout I have been unable to discover that the seg- 
ments are arranged in any definite manner, and have not 
found it possible to distinguish the slightest difference in the 
contents or in the density of any segments during the process 
of segmentation. In size the segments also appear to me to 
bear no relation the one to the other. 
Segmentation is carried on during the passage of the ovum 
down the Fallopian tube, and is completed by the time the 
former reaches the uterus. 
After the close of segmentation, and when the ovum has 
descended into the uterus, but not until then, the segments 
are clearly divided into two layers. The arrangement is as 
follows:—A single layer of cubical hyaline segments com- 
pletely surrounds, except at one point, an inner mass of 
rounded or polygonal densely granular segments. The gap in 
the outer layer of hyaline segments is filled up by one of the 
granular segments (fig. 20). The cause of this sudden change 
is not absolutely clear, but I would suggest the following hypo- 
thesis as a probable explanation. 
I have little hesitation in stating that not only have the 
outer layer of segments become more hyaline than heretofore, 
but the segments of the inner mass have become denser, and 
