CH. IV] CLEAVAGE OF THE EGG 41 



probably comes entirely, or in part, from the surrounding cells. 

 This cavity gets larger and larger as development proceeds 

 (Fig. 15, A). If an egg be cut open after the thirty-two-cell 

 stage, it will be found that many, perhaps all, of the cells or 

 blastomeres are undergoing division into outer and inner cells 

 (Fig. 15, B). We may speak of this process as a delamination. 

 Before the egg has divided into sixty-four cells, as seen on the 

 surface, this delamination into inner and outer cells has in most 

 cells taken place. 



The cell-divisions now proceed more rapidly and with great 

 irregularity. The rhythm also soon becomes lost, so that 

 while some cells are dividing others are resting. Not only 

 have the outer blastomeres continued to divide at the surface, 

 but also below the surface of the egg new blastomeres are being 

 cut off from the outer cells ; the inner blastomeres also continue 

 to divide. In the upper part of the egg a large segmentation- 

 cavity forms. Its roof is covered by several layers of small 

 deeply pigmented cells, its sides by larger cells, and its floor is 

 formed by the large whitish yolk-bearing cells (Fig. 15, C). 



If the surface of the egg be carefully examined during these 

 later stages, it will be found that the cells over one side are dis- 

 tinctly smaller than those over the opposite side. We see that 

 the side of the egg containing the most pigment is made up of 

 larger cells. In Fig. 12, G, H, the opposite sides of an egg are 

 shown, and here the less pigmented cells are seen to be smaller 

 than the cells in the same position on the other side of the egg. 

 Sections show, moreover, that this difference in size is not only 

 found on the surface of the egg, but also in the interior as well. 

 During the early periods of cleavage the egg has become 

 neither more nor less pigmented on its surface, and has retained 

 the same distribution of pigment as in the unsegmented egg. 



Besides the variations in the cleavage noted above, others are 

 more rarely found that depart much further from the usual 

 typical forms. The first furrow, for instance, may divide the 

 egg into very unequal parts. The second furrow may appear 

 before the first has reached the lower pole. The third furrows 

 may stand vertically, passing from near the upper pole into the 

 lower hemisphere, i.e. the third furrows occupy the position of 

 the fourth furrows of the usual type of cleavage. 



