90 CLEAVAGE 



gray crescent. After the first cleavage and up to the late blastula stage, 

 each cell of the embryo is known as a blastomere. 



Before the first cleavage furrow has completely encircled the egg, a 

 second furrow begins at the center of the animal hemisphere and at 

 right angles to the first furrow. This cleavage is also vertical and will 

 progress down and around the egg to become deeper and deeper, but 

 always deepest at the animal hemisphere. This second cleavage first 

 begins to appear at about SVi hours after fertilization, or 1 hour after 

 the completion of the first cleavage. Again the so-called tension lines 

 appear, and, since the lines of the first furrow have by this time flat- 



The 4-cell stage. (A) Animal pole view, to show second 

 cleavage at right angles to the first. (B) Lateral view of 

 the 4-cell stage showing incomplete encirclement of the 

 vegetal hemisphere of the second cleavage furrow. 



tened out, the presence of these new lines plus the relative shallowness 

 of the furrow will help in its identification as the second furrow. As 

 in the case of the first, this second furrow progresses superficially 

 around the egg until the two ends meet at the vegetal pole. In the 

 meantime the first cleavage furrow has cut more deeply into the egg 

 so that in transverse (horizontal) sections of such an egg one can 

 identify the two cleavage furrows easily by their relative depth. 



We can picture the egg at this stage as a core of yolk, concentrated 

 more toward the vegetal pole, and with the cytoplasm, nuclei, and 

 deepest portions of the cleavage furrows located at the pigmented 

 animal hemisphere. However, all of the animal hemisphere structures 

 will spread progressively toward the vegetal pole. With each division 

 of the cytoplasm, cells are formed which contain yolk. By the time 

 the third cleavage makes its appearance, the first cleavage has all but 

 cut the entire egg mass into two equal parts. Each of these first two 

 blastomeres (and the subsequent four blastomeres) is identical with 

 all of the others with respect to the cytoplasm, pigment, and yolk 



