210 WALTER HEAPB. 



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. 313.) 



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. 13 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. 213. 



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 



