i 1 HE SEGMENTATION OF THE OVUM. 



from each other, a second vertical furrow appears at right angles 

 to the first and behaves in the same way (fig. 40, 4). 



The next furrow is equatorial or horizontal (fig. 40, 8). It 

 does not arise at tJie true equator of the egg, but much nearer 

 to its upper pole. It extends rapidly round the egg and divides 

 each of the four previous segments into two parts, one larger and 

 one smaller. Thus at the end of this stage there are present four small 

 and four large segments. At the meeting point of these a 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 succeeding 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, 1 6). A pause now ensues, 

 after which the eight upper seg- 

 ments become divided by an equa- 

 torial furrow, and somewhat later 

 a similar furrow divides the eight 

 lower segments. At the end of 

 this stage there are therefore pre- 

 sent 16 smaller and 16 larger seg- 

 ments (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 segmen- 

 tation 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 vertical furrows 



FIG. 41. SECTION THROUGH FROG'S OVUM 

 AT THE CLOSE OF SEGMENTATION. S(J. S6g- 

 meutatiou cavity. //. large yolk-containing 

 cells, ep. small cells at formative pole 

 (epiblast). 



