CLEAVAGE 9 1 



gradients. In the 4-cell stage they are not qualitatively identical, for 

 only two of the four cells contain any of the gray crescent material. 



Often in closely packed eggs these first two cleavages may appear 

 not quite in the center of the animal hemisphere, or may be not at 

 right angles to each other. This would be the exception, however, and 

 generally is due to the physical compression of the closely packed eggs 

 whereby (as pointed out by Sachs and Hertwig) the cytoplasmic axis 

 is shifted. The description given here applies to the isolated and un- 

 restricted egg and should be considered as the more nearly normal. 

 However, it must be pointed out that this minor compression and 

 shifting of cleavage planes does not affect the normal development of 

 the embryo, provided the distortion is not too great. 



The third cleavage begins about 30 minutes after the second is com- 

 pleted, or 4 hours after fertilization. The cleavage rate is accelerated 

 with each division, possibly because the resulting blastomeres are 

 progressively smaller. The third cleavage plane is horizontal (lati- 

 tudinal ) and at right angles to both the first and the second cleavages. 

 It is slightly above the level of the equator, about 60° to 70° from the 

 center of the animal hemisphere. This shifting of the cleavage plane 

 toward the animal hemisphere from the equator of the otherwise 

 equatorial cleavage is due, no doubt, to the presence of more cytoplasm 

 in that region and to relatively less yolk. Yolk is generally resistant 

 to cleavage forces. When the cleavages are completed, the uppermost 

 and smaller cells are sometimes designated as micromeres (smaller 

 blastomeres) and the lowermost and larger cells are then designated as 

 macromeres (larger blastomeres). This third cleavage furrow is seen 

 as a horizontal groove varying from 20° to 30° above the equatorial 

 line, toward the animal hemisphere. It provides four clear-cut blas- 

 tomeres in the animal hemisphere, each equally pigmented. Since the 

 second cleavage may not have been completed at the vegetal pole, 

 there is a brief transitional period in which from some aspects there 

 appear to be the surface markings of 6 rather than 8 cells. 



The fourth cleavage is usually double and occurs about 20 minutes 

 after the third. It tends to be vertical, but is seldom meridional. The 

 furrows begin near the center of the animal pole and progress vegetally, 

 dividing each of the original four blastomeres of the animal hemi- 

 sphere. Instead of intersecting at the center of the animal hemisphere 

 the fourth cleavage furrows may run into the first or the second 

 cleavage furrows. This cleavage shows the first digression from a regu- 



