THE SEGMENTATION OF THE OVUM. 97 



small cavity appears, which is the segmentation cavity, already 

 described for uniformly segmenting eggs. It increases in size in 

 subsequent stages, its roof being formed of the smaller cells and 

 its floor of the larger. The appearance of the equatorial furrow 

 is followed by a period of repose, after which two rapidly suc- 

 ceeding vertical furrows are formed in the upper pole, dividing 

 each of the four segments of which this is composed into two. 

 After a short period these furrows extend to the lower pole, 

 and when completed 16 segments are present eight larger and 

 eight smaller (fig. 40, 16). A pause now ensues, after which the 

 eight upper segments become divided by an equatorial furrow, 

 and somewhat later a similar furrow divides the eight lower seg- 

 ments. At the end of this stage there are therefore present 16 

 smaller and 16 larger segments (fig. 40, 32). After 64 segments 

 have been formed by vertical furrows which arise symmetrically 

 in the two poles (fig. 40, 64), two equatorial furrows appear in the 

 upper pole before a fresh furrow arises in the lower ; so that there 

 are 128 segments in the upper half, and only 32 in the lower. 

 The regularity is quite lost in subsequent stages, but the upper 

 pole continues to undergo a more rapid segmentation than the 

 lower. While the segments have been increasing in number the 

 segmentation cavity has been rapidly growing in size ; and at the 

 close of segmentation the egg forms a sphere, containing an 

 excentric cavity, and composed of two unequal parts (fig. 41). 

 The upper part, which forms the roof of the segmentation cavity, 

 is formed of smaller cells : the lower of larger yolk-containing 

 cells. 



The mode of segmentation of the Frog's ovum is typical for 

 unequally segmenting ova, and it deserves to be noticed that as 

 regards the first three or more furrows the segmentation occurs 

 with the same rhythm in the unequally segmenting ova as in those 

 which have an uniform segmentation. There appear two verti- 

 cal furrows followed by an equatorial furrow. The general laws 

 which were stated with reference to the velocity of segmentation 

 and the size of the resulting segments are well exemplified in the 

 case of the Frog's ovum. 



The majority of the smaller segments in the segmented Frog's 

 ovum are destined to form into the epiblast, and the larger seg- 

 ments become hypoblast and mesoblast. 



B. ii. 7 



